RFC3435 - Media Gateway Control Protocol (MGCP) Version 1.0
Network Working Group F. Andreasen
Request for Comments: 3435 B. Foster
Obsoletes: 2705 Cisco Systems
Category: Informational January 2003
Media Gateway Control Protocol (MGCP)
Version 1.0
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
IESG Note
This document is being published for the information of the
community. It describes a protocol that is currently being deployed
in a number of prodUCts. Implementers should be aware of RFC3015,
which was developed in the IETF Megaco Working Group and the ITU-T
SG16 and which is considered by the IETF and ITU-T to be the
standards-based (including reviewed security considerations) way to
meet the needs that MGCP was designed to address.
Abstract
This document describes an application programming interface and a
corresponding protocol (MGCP) which is used between elements of a
decomposed multimedia gateway. The decomposed multimedia gateway
consists of a Call Agent, which contains the call control
"intelligence", and a media gateway which contains the media
functions, e.g., conversion from TDM voice to Voice over IP.
Media gateways contain endpoints on which the Call Agent can create,
modify and delete connections in order to establish and control media
sessions with other multimedia endpoints. Also, the Call Agent can
instruct the endpoints to detect certain events and generate signals.
The endpoints automatically communicate changes in service state to
the Call Agent. Furthermore, the Call Agent can audit endpoints as
well as the connections on endpoints.
The basic and general MGCP protocol is defined in this document,
however most media gateways will need to implement one or more MGCP
packages, which define extensions to the protocol suitable for use
with specific types of media gateways. Such packages are defined in
separate documents.
Table of Contents
1. Introduction.................................................5
1.1 Relation with the H.323 Standards............................7
1.2 Relation with the IETF Standards.............................8
1.3 Definitions..................................................9
1.4 Conventions used in this Document............................9
2. Media Gateway Control Interface.............................10
2.1 Model and Naming Conventions................................10
2.1.1 Types of Endpoints..........................................10
2.1.2 Endpoint Identifiers........................................14
2.1.3 Calls and Connections.......................................16
2.1.4 Names of Call Agents and Other Entities.....................22
2.1.5 Digit Maps..................................................23
2.1.6 Packages....................................................26
2.1.7 Events and Signals..........................................28
2.2 Usage of SDP................................................33
2.3 Gateway Control Commands....................................33
2.3.1 Overview of Commands........................................33
2.3.2 EndpointConfiguration.......................................36
2.3.3 NotificationRequest.........................................37
2.3.4 Notify......................................................44
2.3.5 CreateConnection............................................46
2.3.6 ModifyConnection............................................52
2.3.7 DeleteConnection (from the Call Agent)......................54
2.3.8 DeleteConnection (from the gateway).........................58
2.3.9 DeleteConnection (multiple connections from the Call Agent) 59
2.3.10 AuditEndpoint...............................................60
2.3.11 AuditConnection.............................................65
2.3.12 RestartInProgress...........................................66
2.4 Return Codes and Error Codes................................69
2.5 Reason Codes................................................74
2.6 Use of Local Connection Options and Connection Descriptors..75
2.7 Resource Reservations.......................................77
3. Media Gateway Control Protocol..............................77
3.1 General Description.........................................78
3.2 Command Header..............................................79
3.2.1 Command Line................................................79
3.2.2 Parameter Lines.............................................82
3.3 Format of response headers.................................101
3.3.1 CreateConnection Response..................................104
3.3.2 ModifyConnection Response..................................105
3.3.3 DeleteConnection Response..................................106
3.3.4 NotificationRequest Response...............................106
3.3.5 Notify Response............................................106
3.3.6 AuditEndpoint Response.....................................106
3.3.7 AuditConnection Response...................................107
3.3.8 RestartInProgress Response.................................108
3.4 Encoding of the Session Description (SDP)..................108
3.4.1 Usage of SDP for an Audio Service..........................110
3.4.2 Usage of SDP for LOCAL Connections.........................110
3.5 Transmission over UDP......................................111
3.5.1 Providing the At-Most-Once Functionality...................112
3.5.2 Transaction Identifiers and Three Ways Handshake...........113
3.5.3 Computing Retransmission Timers............................114
3.5.4 Maximum Datagram Size, Fragmentation and Reassembly........115
3.5.5 Piggybacking...............................................116
3.5.6 Provisional Responses......................................117
4. States, Failover and Race Conditions.......................119
4.1 Failover Assumptions and Highlights........................119
4.2 Communicating with Gateways................................121
4.3 Retransmission, and Detection of Lost Associations:........122
4.4 Race Conditions............................................126
4.4.1 Quarantine List............................................127
4.4.2 EXPlicit Detection.........................................133
4.4.3 Transactional Semantics....................................134
4.4.4 Ordering of Commands, and Treatment of Misorder............135
4.4.5 Endpoint Service States....................................137
4.4.6 Fighting the Restart Avalanche.............................140
4.4.7 Disconnected Endpoints.....................................143
4.4.8 Load Control in General....................................146
5. Security Requirements......................................147
5.1 Protection of Media Connections............................148
6. Packages...................................................148
6.1 Actions....................................................150
6.2 BearerInformation..........................................150
6.3 ConnectionModes............................................151
6.4 ConnectionParameters.......................................151
6.5 DigitMapLetters............................................151
6.6 Events and Signals.........................................152
6.6.1 Default and Reserved Events................................155
6.7 ExtensionParameters........................................156
6.8 LocalConnectionOptions.....................................157
6.9 Reason Codes...............................................157
6.10 RestartMethods.............................................158
6.11 Return Codes...............................................158
7. Versions and Compatibility.................................158
7.1 Changes from RFC2705......................................158
8. Security Considerations....................................164
9. Acknowledgments............................................164
10. References.................................................164
Appendix A: Formal Syntax Description of the Protocol.............167
Appendix B: Base Package..........................................175
B.1 Events.....................................................175
B.2 Extension Parameters.......................................176
B.2.1 PersistentEvents...........................................176
B.2.2 NotificationState..........................................177
B.3 Verbs......................................................177
Appendix C: IANA Considerations...................................179
C.1 New MGCP Package Sub-Registry..............................179
C.2 New MGCP Package...........................................179
C.3 New MGCP LocalConnectionOptions Sub-Registry...............179
Appendix D: Mode Interactions.....................................180
Appendix E: Endpoint Naming Conventions...........................182
E.1 Analog Access Line Endpoints...............................182
E.2 Digital Trunks.............................................182
E.3 Virtual Endpoints..........................................183
E.4 Media Gateway..............................................184
E.5 Range Wildcards............................................184
Appendix F: Example Command Encodings.............................185
F.1 NotificationRequest........................................185
F.2 Notify.....................................................186
F.3 CreateConnection...........................................186
F.4 ModifyConnection...........................................189
F.5 DeleteConnection (from the Call Agent).....................189
F.6 DeleteConnection (from the gateway)........................190
F.7 DeleteConnection (multiple connections
from the Call Agent).......................................190
F.8 AuditEndpoint..............................................191
F.9 AuditConnection............................................192
F.10 RestartInProgress..........................................193
Appendix G: Example Call Flows....................................194
G.1 Restart....................................................195
G.1.1 Residential Gateway Restart................................195
G.1.2 Call Agent Restart.........................................198
G.2 Connection Creation........................................200
G.2.1 Residential Gateway to Residential Gateway.................200
G.3 Connection Deletion........................................206
G.3.1 Residential Gateway to Residential Gateway.................206
Authors' Addresses................................................209
Full Copyright Statement..........................................210
1. Introduction
This document describes an abstract application programming interface
(MGCI) and a corresponding protocol (MGCP) for controlling media
gateways from external call control elements called media gateway
controllers or Call Agents. A media gateway is typically a network
element that provides conversion between the audio signals carried on
telephone circuits and data packets carried over the Internet or over
other packet networks. Examples of media gateways are:
* Trunking gateways, that interface between the telephone network and
a Voice over IP network. Such gateways typically manage a large
number of digital circuits.
* Voice over ATM gateways, which operate much the same way as voice
over IP trunking gateways, except that they interface to an ATM
network.
* Residential gateways, that provide a traditional analog (RJ11)
interface to a Voice over IP network. Examples of residential
gateways include cable modem/cable set-top boxes, xDSL devices, and
broad-band wireless devices.
* Access gateways, that provide a traditional analog (RJ11) or
digital PBX interface to a Voice over IP network. Examples of
access gateways include small-scale voice over IP gateways.
* Business gateways, that provide a traditional digital PBX interface
or an integrated "soft PBX" interface to a Voice over IP network.
* Network Access Servers, that can attach a "modem" to a telephone
circuit and provide data access to the Internet. We expect that in
the future, the same gateways will combine Voice over IP services
and Network Access services.
* Circuit switches, or packet switches, which can offer a control
interface to an external call control element.
MGCP assumes a call control architecture where the call control
"intelligence" is outside the gateways and handled by external call
control elements known as Call Agents. The MGCP assumes that these
call control elements, or Call Agents, will synchronize with each
other to send coherent commands and responses to the gateways under
their control. If this assumption is violated, inconsistent behavior
should be expected. MGCP does not define a mechanism for
synchronizing Call Agents. MGCP is, in essence, a master/slave
protocol, where the gateways are expected to execute commands sent by
the Call Agents. In consequence, this document specifies in great
detail the expected behavior of the gateways, but only specifies
those parts of a Call Agent implementation, such as timer management,
that are mandated for proper operation of the protocol.
MGCP assumes a connection model where the basic constructs are
endpoints and connections. Endpoints are sources and/or sinks of
data and can be physical or virtual. Examples of physical endpoints
are:
* An interface on a gateway that terminates a trunk connected to a
PSTN switch (e.g., Class 5, Class 4, etc.). A gateway that
terminates trunks is called a trunking gateway.
* An interface on a gateway that terminates an analog POTS connection
to a phone, key system, PBX, etc. A gateway that terminates
residential POTS lines (to phones) is called a residential gateway.
An example of a virtual endpoint is an audio source in an audio-
content server. Creation of physical endpoints requires hardware
installation, while creation of virtual endpoints can be done by
software.
Connections may be either point to point or multipoint. A point to
point connection is an association between two endpoints with the
purpose of transmitting data between these endpoints. Once this
association is established for both endpoints, data transfer between
these endpoints can take place. A multipoint connection is
established by connecting the endpoint to a multipoint session.
Connections can be established over several types of bearer networks,
for example:
* Transmission of audio packets using RTP and UDP over an IP network.
* Transmission of audio packets using AAL2, or another adaptation
layer, over an ATM network.
* Transmission of packets over an internal connection, for example
the TDM backplane or the interconnection bus of a gateway. This is
used, in particular, for "hairpin" connections, connections that
terminate in a gateway but are immediately rerouted over the
telephone network.
For point-to-point connections the endpoints of a connection could be
in separate gateways or in the same gateway.
1.1 Relation with the H.323 Standards
MGCP is designed as an internal protocol within a distributed system
that appears to the outside as a single VoIP gateway. This system is
composed of a Call Agent, that may or may not be distributed over
several computer platforms, and of a set of gateways, including at
least one "media gateway" that perform the conversion of media
signals between circuits and packets, and at least one "signaling
gateway" when connecting to an SS7 controlled network. In a typical
configuration, this distributed gateway system will interface on one
side with one or more telephony (i.e., circuit) switches, and on the
other side with H.323 conformant systems, as indicated in the
following table:
------------------------------------------------------------------
Functional Phone Terminating H.323 conformant
Plane switch Entity systems
---------------------------------------------------------------
Signaling Signaling Call agent Signaling exchanges
Plane exchanges with the Call Agent
through through H.225/RAS and
SS7/ISUP H.225/Q.931.
---------------------------------------------------------------
Possible negotiation
of logical channels
and transmission
parameters through
H.245 with the call
agent.
---------------------------------------------------------------
Internal
synchronization
through MGCP
---------------------------------------------------------------
Bearer Connection Telephony Transmission of VoIP
Data through gateways data using RTP
Transport high speed directly between the
Plane trunk H.323 station and the
groups gateway.
------------------------------------------------------------------
In the MGCP model, the gateways focus on the audio signal translation
function, while the Call Agent handles the call signaling and call
processing functions. As a consequence, the Call Agent implements
the "signaling" layers of the H.323 standard, and presents itself as
an "H.323 Gatekeeper" or as one or more "H.323 Endpoints" to the
H.323 systems.
1.2 Relation with the IETF Standards
While H.323 is the recognized standard for VoIP terminals, the IETF
has also produced specifications for other types of multi-media
applications. These other specifications include:
* the Session Description Protocol (SDP), RFC2327
* the Session Announcement Protocol (SAP), RFC2974
* the Session Initiation Protocol (SIP), RFC3261
* the Real Time Streaming Protocol (RTSP), RFC2326.
The latter three specifications are in fact alternative signaling
standards that allow for the transmission of a session description to
an interested party. SAP is used by multicast session managers to
distribute a multicast session description to a large group of
recipients, SIP is used to invite an individual user to take part in
a point-to-point or unicast session, RTSP is used to interface a
server that provides real time data. In all three cases, the session
description is described according to SDP; when audio is transmitted,
it is transmitted through the Real-time Transport Protocol, RTP.
The distributed gateway systems and MGCP will enable PSTN telephony
users to access sessions set up using SAP, SIP or RTSP. The Call
Agent provides for signaling conversion, according to the following
table:
------------------------------------------------------------------
Functional Phone Terminating IETF conforming systems
Plane switch Entity
---------------------------------------------------------------
Signaling Signaling Call agent Signaling exchanges
Plane exchanges with the Call Agent
through through SAP, SIP or
SS7/ISUP RTSP.
---------------------------------------------------------------
Negotiation of session
description parameters
through SDP (telephony
gateway terminated but
passed via the call
agent to and from the
IETF conforming system)
---------------------------------------------------------------
Internal syn-
chronization
through MGCP
---------------------------------------------------------------
Bearer Connection Telephony Transmission of VoIP
Data through gateways data using RTP,
Transport high speed directly between the
Plane trunk remote IP end system
groups and the gateway.
------------------------------------------------------------------
The SDP standard has a pivotal status in this architecture. We will
see in the following description that we also use it to carry session
descriptions in MGCP.
1.3 Definitions
Trunk: A communication channel between two switching systems, e.g.,
a DS0 on a T1 or E1 line.
1.4 Conventions used in this Document
The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED, "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14, RFC2119 [2].
2. Media Gateway Control Interface
The interface functions provide for connection control and endpoint
control. Both use the same system model and the same naming
conventions.
2.1 Model and Naming Conventions
The MGCP assumes a connection model where the basic constructs are
endpoints and connections. Connections are grouped in calls. One or
more connections can belong to one call. Connections and calls are
set up at the initiative of one or more Call Agents.
2.1.1 Types of Endpoints
In the introduction, we presented several classes of gateways. Such
classifications, however, can be misleading. Manufacturers can
arbitrarily decide to provide several types of services in a single
package. A single product could well, for example, provide some
trunk connections to telephony switches, some primary rate
connections and some analog line interfaces, thus sharing the
characteristics of what we described in the introduction as
"trunking", "access" and "residential" gateways. MGCP does not make
assumptions about such groupings. We simply assume that media
gateways support collections of endpoints. The type of the endpoint
determines its functionality. Our analysis, so far, has led us to
isolate the following basic endpoint types:
* Digital channel (DS0),
* Analog line,
* Announcement server access point,
* Interactive Voice Response access point,
* Conference bridge access point,
* Packet relay,
* ATM "trunk side" interface.
In this section, we will describe the expected behavior of such
endpoints.
This list is not final. There may be other types of endpoints
defined in the future, for example test endpoints that could be used
to check network quality, or frame-relay endpoints that could be used
to manage audio channels multiplexed over a frame-relay virtual
circuit.
2.1.1.1 Digital Channel (DS0)
Digital channels provide a 64 Kbps service. Such channels are found
in trunk and ISDN interfaces. They are typically part of digital
multiplexes, such as T1, E1, T3 or E3 interfaces. Media gateways
that support such channels are capable of translating the digital
signals received on the channel, which may be encoded according to
A-law or mu-law, using either the complete set of 8 bits per sample
or only 7 of these bits, into audio packets. When the media gateway
also supports a Network Access Server (NAS) service, the gateway
shall be capable of receiving either audio-encoded data (modem
connection) or binary data (ISDN connection) and convert them into
data packets.
+-------
+------------+
(channel) ===DS0 endpoint -------- Connections
+------------+
+-------
Media gateways should be able to establish several connections
between the endpoint and the packet networks, or between the endpoint
and other endpoints in the same gateway. The signals originating
from these connections shall be mixed according to the connection
"mode", as specified later in this document. The precise number of
connections that an endpoint supports is a characteristic of the
gateway, and may in fact vary according to the allocation of
resources within the gateway.
In some cases, digital channels are used to carry signaling. This is
the case for example for SS7 "F" links, or ISDN "D" channels. Media
gateways that support these signaling functions shall be able to send
and receive the signaling packets to and from a Call Agent, using the
"backhaul" procedures defined by the SIGTRAN working group of the
IETF. Digital channels are sometimes used in conjunction with
channel associated signaling, such as "MF R2". Media gateways that
support these signaling functions shall be able to detect and produce
the corresponding signals, such as for example "wink" or "A",
according to the event signaling and reporting procedures defined in
MGCP.
2.1.1.2 Analog Line
Analog lines can be used either as a "client" interface, providing
service to a classic telephone unit, or as a "service" interface,
allowing the gateway to send and receive analog calls. When the
media gateway also supports a NAS service, the gateway shall be
capable of receiving audio-encoded data (modem connection) and
convert them into data packets.
+-------
+---------------+
(line) ===analog endpoint -------- Connections
+---------------+
+-------
Media gateways should be able to establish several connections
between the endpoint and the packet networks, or between the endpoint
and other endpoints in the same gateway. The audio signals
originating from these connections shall be mixed according to the
connection "mode", as specified later in this document. The precise
number of connections that an endpoint supports is a characteristic
of the gateway, and may in fact vary according to the allocation of
resources within the gateway. A typical gateway should however be
able to support two or three connections per endpoint, in order to
support services such as "call waiting" or "three way calling".
2.1.1.3 Announcement Server Access Point
An announcement server endpoint provides access to an announcement
service. Under requests from the Call Agent, the announcement server
will "play" a specified announcement. The requests from the Call
Agent will follow the event signaling and reporting procedures
defined in MGCP.
+----------------------+
Announcement endpoint -------- Connection
+----------------------+
A given announcement endpoint is not expected to support more than
one connection at a time. If several connections were established to
the same endpoint, then the same announcements would be played
simultaneously over all the connections.
Connections to an announcement server are typically one way, or "half
duplex" -- the announcement server is not expected to listen to the
audio signals from the connection.
2.1.1.4 Interactive Voice Response Access Point
An Interactive Voice Response (IVR) endpoint provides access to an
IVR service. Under requests from the Call Agent, the IVR server will
"play" announcements and tones, and will "listen" to responses, such
as DTMF input or voice messages, from the user. The requests from
the Call Agent will follow the event signaling and reporting
procedures defined in MGCP.
+-------------+
IVR endpoint -------- Connection
+-------------+
A given IVR endpoint is not expected to support more than one
connection at a time. If several connections were established to the
same endpoint, then the same tones and announcements would be played
simultaneously over all the connections.
2.1.1.5 Conference Bridge Access Point
A conference bridge endpoint is used to provide access to a specific
conference.
+-------
+--------------------------+
Conference bridge endpoint -------- Connections
+--------------------------+
+-------
Media gateways should be able to establish several connections
between the endpoint and the packet networks, or between the endpoint
and other endpoints in the same gateway. The signals originating
from these connections shall be mixed according to the connection
"mode", as specified later in this document. The precise number of
connections that an endpoint supports is a characteristic of the
gateway, and may in fact vary according to the allocation of
resources within the gateway.
2.1.1.6 Packet Relay
A packet relay endpoint is a specific form of conference bridge, that
typically only supports two connections. Packets relays can be found
in firewalls between a protected and an open network, or in
transcoding servers used to provide interoperation between
incompatible gateways, for example gateways that do not support
compatible compression algorithms, or gateways that operate over
different transmission networks such as IP and ATM.
+-------
+---------------------+
Packet relay endpoint 2 connections
+---------------------+
+-------
2.1.1.7 ATM "trunk side" Interface
ATM "trunk side" endpoints are typically found when one or several
ATM permanent virtual circuits are used as a replacement for the
classic "TDM" trunks linking switches. When ATM/AAL2 is used,
several trunks or channels are multiplexed on a single virtual
circuit; each of these trunks correspond to a single endpoint.
+-------
+------------------+
(channel) = ATM trunk endpoint -------- Connections
+------------------+
+-------
Media gateways should be able to establish several connections
between the endpoint and the packet networks, or between the endpoint
and other endpoints in the same gateway. The signals originating
from these connections shall be mixed according to the connection
"mode", as specified later in this document. The precise number of
connections that an endpoint supports is a characteristic of the
gateway, and may in fact vary according to the allocation of
resources within the gateway.
2.1.2 Endpoint Identifiers
Endpoint identifiers have two components that both are case-
insensitive:
* the domain name of the gateway that is managing the endpoint
* a local name within that gateway
Endpoint names are of the form:
local-endpoint-name@domain-name
where domain-name is an absolute domain-name as defined in RFC1034
and includes a host portion, thus an example domain-name could be:
mygateway.whatever.net
Also, domain-name may be an IP-address of the form defined for domain
name in RFC821, thus another example could be (see RFC821 for
details):
[192.168.1.2]
Both IPv4 and IPv6 addresses can be specified, however use of IP
addresses as endpoint identifiers is generally discouraged.
Note that since the domain name portion is part of the endpoint
identifier, different forms or different values referring to the same
entity are not freely interchangeable. The most recently supplied
form and value MUST always be used.
The local endpoint name is case-insensitive. The syntax of the local
endpoint name is hierarchical, where the least specific component of
the name is the leftmost term, and the most specific component is the
rightmost term. The precise syntax depends on the type of endpoint
being named and MAY start with a term that identifies the endpoint
type. In any case, the local endpoint name MUST adhere to the
following naming rules:
1) The individual terms of the naming path MUST be separated by a
single slash ("/", ASCII 2F hex).
2) The individual terms are character strings composed of letters,
digits or other printable characters, with the exception of
characters used as delimiters ("/", "@"), characters used for
wildcarding ("*", "$") and white spaces.
3) Wild-carding is represented either by an asterisk ("*") or a
dollar sign ("$") for the terms of the naming path which are to be
wild-carded. Thus, if the full local endpoint name is of the
form:
term1/term2/term3
then the entity name field looks like this depending on which
terms are wild-carded:
*/term2/term3 if term1 is wild-carded
term1/*/term3 if term2 is wild-carded
term1/term2/* if term3 is wild-carded
term1/*/* if term2 and term3 are wild-carded, etc.
In each of these examples a dollar sign could have appeared
instead of an asterisk.
4) A term represented by an asterisk ("*") is to be interpreted as:
"use ALL values of this term known within the scope of the Media
Gateway". Unless specified otherwise, this refers to all
endpoints configured for service, regardless of their actual
service state, i.e., in-service or out-of-service.
5) A term represented by a dollar sign ("$") is to be interpreted as:
"use ANY ONE value of this term known within the scope of the
Media Gateway". Unless specified otherwise, this only refers to
endpoints that are in-service.
Furthermore, it is RECOMMENDED that Call Agents adhere to the
following:
* Wild-carding should only be done from the right, thus if a term is
wild-carded, then all terms to the right of that term should be
wild-carded as well.
* In cases where mixed dollar sign and asterisk wild-cards are used,
dollar-signs should only be used from the right, thus if a term had
a dollar sign wild-card, all terms to the right of that term should
also contain dollar sign wild-cards.
The description of a specific command may add further criteria for
selection within the general rules given above.
Note, that wild-cards may be applied to more than one term in which
case they shall be evaluated from left to right. For example, if we
have the endpoint names "a/1", "a/2", "b/1", and "b/2", then "$/*"
(which is not recommended) will evaluate to either "a/1, a/2", or
"b/1, b/2". However, "*/$" may evaluate to "a/1, b/1", "a/1, b/2",
"a/2, b/1", or "a/2, b/2". The use of mixed wild-cards in a command
is considered error prone and is consequently discouraged.
A local name that is composed of only a wildcard character refers to
either all (*) or any ($) endpoints within the media gateway.
2.1.3 Calls and Connections
Connections are created on the Call Agent on each endpoint that will
be involved in the "call". In the classic example of a connection
between two "DS0" endpoints (EP1 and EP2), the Call Agents
controlling the endpoints will establish two connections (C1 and C2):
+---+ +---+
(channel1) ===EP1--(C1)--... ...(C2)--EP2===(channel2)
+---+ +---+
Each connection will be designated locally by an endpoint unique
connection identifier, and will be characterized by connection
attributes.
When the two endpoints are located on gateways that are managed by
the same Call Agent, the creation is done via the three following
steps:
1) The Call Agent asks the first gateway to "create a connection" on
the first endpoint. The gateway allocates resources to that
connection, and responds to the command by providing a "session
description". The session description contains the information
necessary for a third party to send packets towards the newly
created connection, such as for example IP address, UDP port, and
codec parameters.
2) The Call Agent then asks the second gateway to "create a
connection" on the second endpoint. The command carries the
"session description" provided by the first gateway. The gateway
allocates resources to that connection, and responds to the
command by providing its own "session description".
3) The Call Agent then uses a "modify connection" command to provide
this second "session description" to the first endpoint. Once
this is done, communication can proceed in both directions.
When the two endpoints are located on gateways that are managed by
two different Call Agents, the Call Agents exchange information
through a Call-Agent to Call-Agent signaling protocol, e.g., SIP [7],
in order to synchronize the creation of the connection on the two
endpoints.
Once a connection has been established, the connection parameters can
be modified at any time by a "modify connection" command. The Call
Agent may for example instruct the gateway to change the codec used
on a connection, or to modify the IP address and UDP port to which
data should be sent, if a connection is "redirected".
The Call Agent removes a connection by sending a "delete connection"
command to the gateway. The gateway may also, under some
circumstances, inform a gateway that a connection could not be
sustained.
The following diagram provides a view of the states of a connection,
as seen from the gateway:
Create connection
received
V
+-------------------+
resource allocation-(failed)-+
+-------------------+
(connection refused)
(successful)
v
+----------->+
+-------------------+
remote session
description ----------(yes)--------+
available ?
+-------------------+
(no)
+-----------+ +------+
+---> half open ------> Delete <------- open <----------+
(wait) Connection (wait)
+-----------+ received +------+
Modify Connection Modify Connection
received received
+--------------------+ +--------------------+
assess modification assess modification
+--------------------+ +--------------------+
(failed) (successful) (failed) (successful)
+<---+ +-------------+-------+
+<-------------------+
+-----------------+
Free connection
resources.
Report.
+-----------------+
V
2.1.3.1 Names of Calls
One of the attributes of each connection is the "call identifier",
which as far as the MGCP protocol is concerned has little semantic
meaning, and is mainly retained for backwards compatibility.
Calls are identified by unique identifiers, independent of the
underlying platforms or agents. Call identifiers are hexadecimal
strings, which are created by the Call Agent. The maximum length of
call identifiers is 32 characters.
Call identifiers are expected to be unique within the system, or at a
minimum, unique within the collection of Call Agents that control the
same gateways. From the gateway's perspective, the Call identifier
is thus unique. When a Call Agent builds several connections that
pertain to the same call, either on the same gateway or in different
gateways, these connections that belong to the same call should share
the same call-id. This identifier can then be used by accounting or
management procedures, which are outside the scope of MGCP.
2.1.3.2 Names of Connections
Connection identifiers are created by the gateway when it is
requested to create a connection. They identify the connection
within the context of an endpoint. Connection identifiers are
treated in MGCP as hexadecimal strings. The gateway MUST make sure
that a proper waiting period, at least 3 minutes, elapses between the
end of a connection that used this identifier and its use in a new
connection for the same endpoint (gateways MAY decide to use
identifiers that are unique within the context of the gateway). The
maximum length of a connection identifier is 32 characters.
2.1.3.3 Management of Resources, Attributes of Connections
Many types of resources will be associated to a connection, such as
specific signal processing functions or packetization functions.
Generally, these resources fall in two categories:
1) Externally visible resources, that affect the format of "the bits
on the network" and must be communicated to the second endpoint
involved in the connection.
2) Internal resources, that determine which signal is being sent over
the connection and how the received signals are processed by the
endpoint.
The resources allocated to a connection, and more generally the
handling of the connection, are chosen by the gateway under
instructions from the Call Agent. The Call Agent will provide these
instructions by sending two sets of parameters to the gateway:
1) The local directives instruct the gateway on the choice of
resources that should be used for a connection,
2) When available, the "session description" provided by the other
end of the connection (referred to as the remote session
description).
The local directives specify such parameters as the mode of the
connection (e.g., send-only, or send-receive), preferred coding or
packetization methods, usage of echo cancellation or silence
suppression. (A detailed list can be found in the specification of
the LocalConnectionOptions parameter of the CreateConnection
command.) Depending on the parameter, the Call Agent MAY either
specify a value, a range of values, or no value at all. This allows
various implementations to implement various levels of control, from
a very tight control where the Call Agent specifies minute details of
the connection handling to a very loose control where the Call Agent
only specifies broad guidelines, such as the maximum bandwidth, and
lets the gateway choose the detailed values subject to the
guidelines.
Based on the value of the local directives, the gateway will
determine the resources to allocate to the connection. When this is
possible, the gateway will choose values that are in line with the
remote session description - but there is no absolute requirement
that the parameters be exactly the same.
Once the resources have been allocated, the gateway will compose a
"session description" that describes the way it intends to send and
receive packets. Note that the session description may in some cases
present a range of values. For example, if the gateway is ready to
accept one of several compression algorithms, it can provide a list
of these accepted algorithms.
Local Directives
(from Call Agent 1)
V
+-------------+
resource
allocation
(gateway 1)
+-------------+
V
Local
Parameters V
Session
Description Local Directives
(from Call Agent 2)
+---> Transmission----+
(CA to CA)
V V
+-------------+
resource
allocation
(gateway 2)
+-------------+
V
Local
Parameters
Session
Description
+---- Transmission<---+
(CA to CA)
V V
+-------------+
modification
(gateway 1)
+-------------+
V
Local
Parameters
-- Information flow: local directives & session descriptions --
2.1.3.4 Special Case of Local Connections
Large gateways include a large number of endpoints which are often of
different types. In some networks, we may often have to set-up
connections between endpoints that are located within the same
gateway. Examples of such connections may be:
* Connecting a call to an Interactive Voice-Response unit,
* Connecting a call to a Conferencing unit,
* Routing a call from one endpoint to another, something often
described as a "hairpin" connection.
Local connections are much simpler to establish than network
connections. In most cases, the connection will be established
through some local interconnecting device, such as for example a TDM
bus.
When two endpoints are managed by the same gateway, it is possible to
specify the connection in a single command that conveys the names of
the two endpoints that will be connected. The command is essentially
a "Create Connection" command which includes the name of the second
endpoint in lieu of the "remote session description".
2.1.4 Names of Call Agents and Other Entities
The media gateway control protocol has been designed to allow the
implementation of redundant Call Agents, for enhanced network
reliability. This means that there is no fixed binding between
entities and hardware platforms or network interfaces.
Call Agent names consist of two parts, similar to endpoint names.
Semantically, the local portion of the name does not exhibit any
internal structure. An example Call Agent name is:
ca1@ca.whatever.net
Note that both the local part and the domain name have to be
supplied. Nevertheless, implementations are encouraged to accept call
agent names consisting of only the domain name.
Reliability can be improved by using the following procedures:
* Entities such as endpoints or Call Agents are identified by their
domain name, not their network addresses. Several addresses can be
associated with a domain name. If a command or a response cannot
be forwarded to one of the network addresses, implementations MUST
retry the transmission using another address.
* Entities MAY move to another platform. The association between a
logical name (domain name) and the actual platform is kept in the
domain name service. Call Agents and Gateways MUST keep track of
the time-to-live of the record they read from the DNS. They MUST
query the DNS to refresh the information if the time to live has
expired.
In addition to the indirection provided by the use of domain names
and the DNS, the concept of "notified entity" is central to
reliability and fail-over in MGCP. The "notified entity" for an
endpoint is the Call Agent currently controlling that endpoint. At
any point in time, an endpoint has one, and only one, "notified
entity" associated with it. The "notified entity" determines where
the endpoint will send commands to; when the endpoint needs to send a
command to the Call Agent, it MUST send the command to its current
"notified entity". The "notified entity" however does not determine
where commands can be received from; any Call Agent can send commands
to the endpoint. Please refer to Section 5 for the relevant security
considerations.
Upon startup, the "notified entity" MUST be set to a provisioned
value. Most commands sent by the Call Agent include the ability to
explicitly name the "notified entity" through the use of a
"NotifiedEntity" parameter. The "notified entity" will stay the same
until either a new "NotifiedEntity" parameter is received or the
endpoint does a warm or cold (power-cycle) restart.
If a "NotifiedEntity" parameter is sent with an "empty" value, the
"notified entity" for the endpoint will be set to empty. If the
"notified entity" for an endpoint is empty or has not been set
explicitly (neither by a command nor by provisioning), the "notified
entity" will then default to the source address (i.e., IP address and
UDP port number) of the last successful non-audit command received
for the endpoint. Auditing will thus not change the "notified
entity". Use of an empty "NotifiedEntity" parameter value is
strongly discouraged as it is error prone and eliminates the DNS-
based fail-over and reliability mechanisms.
2.1.5 Digit Maps
The Call Agent can ask the gateway to collect digits dialed by the
user. This facility is intended to be used with residential gateways
to collect the numbers that a user dials; it can also be used with
trunking gateways and access gateways alike, to collect access codes,
credit card numbers and other numbers requested by call control
services.
One procedure is for the gateway to notify the Call Agent of each
individual dialed digit, as soon as they are dialed. However, such a
procedure generates a large number of interactions. It is preferable
to accumulate the dialed numbers in a buffer, and to transmit them in
a single message.
The problem with this accumulation approach, however, is that it is
hard for the gateway to predict how many numbers it needs to
accumulate before transmission. For example, using the phone on our
desk, we can dial the following numbers:
------------------------------------------------------
0 Local operator
00 Long distance operator
xxxx Local extension number
8xxxxxxx Local number
#xxxxxxx Shortcut to local number at
other corporate sites
*xx Star services
91xxxxxxxxxx Long distance number
9011 + up to 15 digits International number
------------------------------------------------------
The solution to this problem is to have the Call Agent load the
gateway with a digit map that may correspond to the dial plan. This
digit map is expressed using a syntax derived from the Unix system
command, egrep. For example, the dial plan described above results
in the following digit map:
(0T00T[1-7]xxx8xxxxxxx#xxxxxxx*xx91xxxxxxxxxx9011x.T)
The formal syntax of the digit map is described by the DigitMap rule
in the formal syntax description of the protocol (see Appendix A) -
support for basic digit map letters is REQUIRED while support for
extension digit map letters is OPTIONAL. A gateway receiving a digit
map with an extension digit map letter not supported SHOULD return
error code 537 (unknown digit map extension).
A digit map, according to this syntax, is defined either by a (case
insensitive) "string" or by a list of strings. Each string in the
list is an alternative numbering scheme, specified either as a set of
digits or timers, or as an expression over which the gateway will
attempt to find a shortest possible match. The following constructs
can be used in each numbering scheme:
* Digit: A digit from "0" to "9".
* Timer: The symbol "T" matching a timer expiry.
* DTMF: A digit, a timer, or one of the symbols "A", "B", "C",
"D", "#", or "*". Extensions may be defined.
* Wildcard: The symbol "x" which matches any digit ("0" to "9").
* Range: One or more DTMF symbols enclosed between square brackets
("[" and "]").
* Subrange: Two digits separated by hyphen ("-") which matches any
digit between and including the two. The subrange
construct can only be used inside a range construct,
i.e., between "[" and "]".
* Position: A period (".") which matches an arbitrary number,
including zero, of occurrences of the preceding
construct.
A gateway that detects events to be matched against a digit map MUST
do the following:
1) Add the event code as a token to the end of an internal state
variable for the endpoint called the "current dial string".
2) Apply the current dial string to the digit map table, attempting a
match to each expression in the digit map.
3) If the result is under-qualified (partially matches at least one
entry in the digit map and doesn't completely match another
entry), do nothing further.
If the result matches an entry, or is over-qualified (i.e., no
further digits could possibly produce a match), send the list of
accumulated events to the Call Agent. A match, in this
specification, can be either a "perfect match," exactly matching one
of the specified alternatives, or an impossible match, which occurs
when the dial string does not match any of the alternatives.
Unexpected timers, for example, can cause "impossible matches". Both
perfect matches and impossible matches trigger notification of the
accumulated digits (which may include other events - see Section
2.3.3).
The following example illustrates the above. Assume we have the
digit map:
(xxxxxxxx11)
and a current dial string of "41". Given the input "1" the current
dial string becomes "411". We have a partial match with "xxxxxxx",
but a complete match with "x11", and hence we send "411" to the Call
Agent.
The following digit map example is more suBTle:
(0[12].001[12].12x.#)
Given the input "0", a match will occur immediately since position
(".") allows for zero occurrences of the preceding construct. The
input "00" can thus never be produced in this digit map.
Given the input "1", only a partial match exists. The input "12" is
also only a partial match, however both "11" and "121" are a match.
Given the input "2", a partial match exists. A partial match also
exists for the input "23", "234", "2345", etc. A full match does not
occur here until a "#" is generated, e.g., "2345#". The input "2#"
would also have been a match.
Note that digit maps simply define a way of matching sequences of
event codes against a grammar. Although digit maps as defined here
are for DTMF input, extension packages can also be defined so that
digit maps can be used for other types of input represented by event
codes that adhere to the digit map syntax already defined for these
event codes (e.g., "1" or "T"). Where such usage is envisioned, the
definition of the particular event(s) SHOULD explicitly state that in
the package definition.
Since digit maps are not bounded in size, it is RECOMMENDED that
gateways support digit maps up to at least 2048 bytes per endpoint.
2.1.6 Packages
MGCP is a modular and extensible protocol, however with extensibility
comes the need to manage, identify, and name the individual
extensions. This is achieved by the concept of packages, which are
simply well-defined groupings of extensions. For example, one
package may support a certain group of events and signals, e.g.,
off-hook and ringing, for analog access lines. Another package may
support another group of events and signals for analog access lines
or for another type of endpoint such as video. One or more packages
may be supported by a given endpoint.
MGCP allows the following types of extensions to be defined in a
package:
* BearerInformation
* LocalConnectionOptions
* ExtensionParameters
* ConnectionModes
* Events
* Signals
* Actions
* DigitMapLetters
* ConnectionParameters
* RestartMethods
* ReasonCodes
* Return codes
each of which will be explained in more detail below. The rules for
defining each of these extensions in a package are described in
Section 6, and the encoding and syntax are defined in Section 3 and
Appendix A.
With the exception of DigitMapLetters, a package defines a separate
name space for each type of extension by adding the package name as a
prefix to the extension, i.e.:
package-name/extension
Thus the package-name is followed by a slash ("/") and the name of
the extension.
An endpoint supporting one or more packages may define one of those
packages as the default package for the endpoint. Use of the package
name for events and signals in the default package for an endpoint is
OPTIONAL, however it is RECOMMENDED to always include the package
name. All other extensions, except DigitMapLetter, defined in the
package MUST include the package-name when referring to the
extension.
Package names are case insensitive strings of letters, hyphens and
digits, with the restriction that hyphens shall never be the first or
last character in a name. Examples of package names are "D", "T",
and "XYZ". Package names are not case sensitive - names such as
"XYZ", "xyz", and "xYz" are equal.
Package definitions will be provided in other documents and with
package names and extensions names registered with IANA. For more
details, refer to section 6.
Implementers can gain experience by using experimental packages. The
name of an experimental package MUST start with the two characters
"x-"; the IANA SHALL NOT register package names that start with these
characters, or the characters "x+", which are reserved. A gateway
that receives a command referring to an unsupported package MUST
return an error (error code 518 - unsupported package, is
RECOMMENDED).
2.1.7 Events and Signals
The concept of events and signals is central to MGCP. A Call Agent
may ask to be notified about certain events occurring in an endpoint
(e.g., off-hook events) by including the name of the event in a
RequestedEvents parameter (in a NotificationRequest command - see
Section 2.3.3).
A Call Agent may also request certain signals to be applied to an
endpoint (e.g., dial-tone) by supplying the name of the event in a
SignalRequests parameter.
Events and signals are grouped in packages, within which they share
the same name space which we will refer to as event names in the
following. Event names are case insensitive strings of letters,
hyphens and digits, with the restriction that hyphens SHALL NOT be
the first or last character in a name. Some event codes may need to
be parameterized with additional data, which is accomplished by
adding the parameters between a set of parentheses. Event names are
not case sensitive - values such as "hu", "Hu", "HU" or "hU" are
equal.
Examples of event names can be "hu" (off hook or "hang-up"
transition), "hf" (hook-flash) or "0" (the digit zero).
The package name is OPTIONAL for events in the default package for an
endpoint, however it is RECOMMENDED to always include the package
name. If the package name is excluded from the event name, the
default package name for that endpoint MUST be assumed. For example,
for an analog access line which has the line package ("L") as a
default with dial-tone ("dl") as one of the events in that package,
the following two event names are equal:
L/dl
and
dl
For any other non-default packages that are associated with that
endpoint, (such as the generic package for an analog access
endpoint-type for example), the package name MUST be included with
the event name. Again, unconditional inclusion of the package name
is RECOMMENDED.
Digits, or letters, are supported in some packages, notably "DTMF".
Digits and letters are defined by the rules "Digit" and "Letter" in
the definition of digit maps. This definition refers to the digits
(0 to 9), to the asterisk or star ("*") and orthotrope, number or
pound sign ("#"), and to the letters "A", "B", "C" and "D", as well
as the timer indication "T". These letters can be combined in "digit
string" that represents the keys that a user punched on a dial. In
addition, the letter "X" can be used to represent all digits (0 to
9). Also, extensions MAY define use of other letters. The need to
easily express the digit strings in earlier versions of the protocol
has a consequence on the form of event names:
An event name that does not denote a digit MUST always contain at
least one character that is neither a digit, nor one of the letters
A, B, C, D, T or X (such names also MUST NOT just contain the special
signs "*", or "#"). Event names consisting of more than one
character however may use any of the above.
A Call Agent may often have to ask a gateway to detect a group of
events. Two conventions can be used to denote such groups:
* The "*" and "all" wildcard conventions (see below) can be used to
detect any event belonging to a package, or a given event in many
packages, or any event in any package supported by the gateway.
* The regular expression Range notation can be used to detect a range
of digits.
The star sign (*) can be used as a wildcard instead of a package
name, and the keyword "all" can be used as a wildcard instead of an
event name:
* A name such as "foo/all" denotes all events in package "foo".
* A name such as "*/bar" denotes the event "bar" in any package
supported by the gateway.
* The name "*/all" denotes all events supported by the endpoint.
This specification purposely does not define any additional detail
for the "all packages" and "all events" wildcards. They provide
limited benefits, but introduce significant complexity along with the
potential for errors. Their use is consequently strongly
discouraged.
The Call Agent can ask a gateway to detect a set of digits or letters
either by individually describing those letters, or by using the
"range" notation defined in the syntax of digit strings. For
example, the Call Agent can:
* Use the letter "x" to denote" digits from 0 to 9.
* Use the notation "[0-9#]" to denote the digits 0 to 9 and the pound
sign.
The individual event codes are still defined in a package though
(e.g., the "DTMF" package).
Events can by default only be generated and detected on endpoints,
however events can be also be defined so they can be generated or
detected on connections rather than on the endpoint itself (see
Section 6.6). For example, gateways may be asked to provide a
ringback tone on a connection. When an event is to be applied on a
connection, the name of the connection MUST be added to the name of
the event, using an "at" sign (@) as a delimiter, as in:
G/rt@0A3F58
where "G" is the name of the package and "rt" is the name of the
event. Should the connection be deleted while an event or signal is
being detected or applied on it, that particular event detection or
signal generation simply stops. Depending on the signal, this may
generate a failure (see below).
The wildcard character "*" (star) can be used to denote "all
connections". When this convention is used, the gateway will
generate or detect the event on all the connections that are
connected to the endpoint. This applies to existing as well as
future connections created on the endpoint. An example of this
convention could be:
R/qa@*
where "R" is the name of the package and "qa" is the name of the
event.
When processing a command using the "all connections" wildcard, the
"*" wildcard character applies to all current and future connections
on the endpoint, however it will not be expanded. If a subsequent
command either explicitly (e.g., by auditing) or implicitly (e.g., by
persistence) refers to such an event, the "*" value will be used.
However, when the event is actually observed, that particular
occurrence of the event will include the name of the specific
connection it occurred on.
The wildcard character "$" can be used to denote "the current
connection". It can only be used by the Call Agent, when the event
notification request is "encapsulated" within a connection creation
or modification command. When this convention is used, the gateway
will generate or detect the event on the connection that is currently
being created or modified. An example of this convention is:
G/rt@$
When processing a command using the "current connection" wildcard,
the "$" wildcard character will be expanded to the value of the
current connection. If a subsequent command either explicitly (e.g.,
by auditing) or implicitly (e.g., by persistence) refers to such an
event, the expanded value will be used. In other words, the "current
connection" wildcard is expanded once, which is at the initial
processing of the command in which it was explicitly included.
The connection id, or a wildcard replacement, can be used in
conjunction with the "all packages" and "all events" conventions. For
example, the notation:
*/all@*
can be used to designate all events on all current and future
connections on the endpoint. However, as mentioned before, the use
of the "all packages" and "all events" wildcards are strongly
discouraged.
Signals are divided into different types depending on their behavior:
* On/off (OO): Once applied, these signals last until they are
turned off. This can only happen as the result of a reboot/restart
or a new SignalRequests where the signal is explicitly turned off
(see later). Signals of type OO are defined to be idempotent, thus
multiple requests to turn a given OO signal on (or off) are
perfectly valid and MUST NOT result in any errors. An On/Off
signal could be a visual message-waiting indicator (VMWI). Once
turned on, it MUST NOT be turned off until explicitly instructed to
by the Call Agent, or as a result of an endpoint restart, i.e.,
these signals will not turn off as a result of the detection of a
requested event.
* Time-out (TO): Once applied, these signals last until they are
either cancelled (by the occurrence of an event or by not being
included in a subsequent (possibly empty) list of signals), or a
signal-specific period of time has elapsed. A TO signal that times
out will generate an "operation complete" event. A TO signal could
be "ringback" timing out after 180 seconds. If an event occurs
prior to the 180 seconds, the signal will, by default, be stopped
(the "Keep signals active" action - see Section 2.3.3 - will
override this behavior). If the signal is not stopped, the signal
will time out, stop and generate an "operation complete" event,
about which the Call Agent may or may not have requested to be
notified. If the Call Agent has asked for the "operation complete"
event to be notified, the "operation complete" event sent to the
Call Agent SHALL include the name(s) of the signal(s) that timed
out (note that if parameters were passed to the signal, the
parameters will not be reported). If the signal was generated on a
connection, the name of the connection SHALL be included as
described above. Time-out signals have a default time-out value
defined for them, which MAY be altered by the provisioning process.
Also, the time-out period may be provided as a parameter to the
signal (see Section 3.2.2.4). A value of zero indicates that the
time-out period is infinite. A TO signal that fails after being
started, but before having generated an "operation complete" event
will generate an "operation failure" event which will include the
name of the signal that failed. Deletion of a connection with an
active TO signal will result in such a failure.
* Brief (BR): The duration of these signals is normally so short
that they stop on their own. If a signal stopping event occurs, or
a new SignalRequests is applied, a currently active BR signal will
not stop. However, any pending BR signals not yet applied MUST be
cancelled (a BR signal becomes pending if a NotificationRequest
includes a BR signal, and there is already an active BR signal). As
an example, a brief tone could be a DTMF digit. If the DTMF digit
"1" is currently being played, and a signal stopping event occurs,
the "1" would play to completion. If a request to play DTMF digit
"2" arrives before DTMF digit "1" finishes playing, DTMF digit "2"
would become pending.
Signal(s) generated on a connection MUST include the name of that
connection.
2.2 Usage of SDP
The Call Agent uses the MGCP to provide the endpoint with the
description of connection parameters such as IP addresses, UDP port
and RTP profiles. These descriptions will follow the conventions
delineated in the Session Description Protocol which is now an IETF
proposed standard, documented in RFC2327.
2.3 Gateway Control Commands
2.3.1 Overview of Commands
This section describes the commands of the MGCP. The service
consists of connection handling and endpoint handling commands.
There are currently nine commands in the protocol:
* The Call Agent can issue an EndpointConfiguration command to a
gateway, instructing the gateway about the coding characteristics
expected by the "line-side" of the endpoint.
* The Call Agent can issue a NotificationRequest command to a
gateway, instructing the gateway to watch for specific events such
as hook actions or DTMF tones on a specified endpoint.
* The gateway will then use the Notify command to inform the Call
Agent when the requested events occur.
* The Call Agent can use the CreateConnection command to create a
connection that terminates in an "endpoint" inside the gateway.
* The Call Agent can use the ModifyConnection command to change the
parameters associated with a previously established connection.
* The Call Agent can use the DeleteConnection command to delete an
existing connection. The DeleteConnection command may also be used
by a gateway to indicate that a connection can no longer be
sustained.
* The Call Agent can use the AuditEndpoint and AuditConnection
commands to audit the status of an "endpoint" and any connections
associated with it. Network management beyond the capabilities
provided by these commands is generally desirable. Such
capabilities are expected to be supported by the use of the Simple
Network Management Protocol (SNMP) and definition of a MIB which is
outside the scope of this specification.
* The Gateway can use the RestartInProgress command to notify the
Call Agent that a group of endpoints managed by the gateway is
being taken out-of-service or is being placed back in-service.
These services allow a controller (normally, the Call Agent) to
instruct a gateway on the creation of connections that terminate in
an "endpoint" attached to the gateway, and to be informed about
events occurring at the endpoint. An endpoint may be for example:
* A specific trunk circuit, within a trunk group terminating in a
gateway,
* A specific announcement handled by an announcement server.
Connections are logically grouped into "calls" (the concept of a
"call" has however little semantic meaning in MGCP itself). Several
connections, that may or may not belong to the same call, can
terminate in the same endpoint. Each connection is qualified by a
"mode" parameter, which can be set to "send only" (sendonly),
"receive only" (recvonly), "send/receive" (sendrecv), "conference"
(confrnce), "inactive" (inactive), "loopback", "continuity test"
(conttest), "network loop back" (netwloop) or "network continuity
test" (netwtest).
Media generated by the endpoint is sent on connections whose mode is
either "send only", "send/receive", or "conference", unless the
endpoint has a connection in "loopback" or "continuity test" mode.
However, media generated by applying a signal to a connection is
always sent on the connection, regardless of the mode.
The handling of the media streams received on connections is
determined by the mode parameters:
* Media streams received through connections in "receive",
"conference" or "send/receive" mode are mixed and sent to the
endpoint, unless the endpoint has another connection in "loopback"
or "continuity test" mode.
* Media streams originating from the endpoint are transmitted over
all the connections whose mode is "send", "conference" or
"send/receive", unless the endpoint has another connection in
"loopback" or "continuity test" mode.
* In addition to being sent to the endpoint, a media stream received
through a connection in "conference" mode is forwarded to all the
other connections whose mode is "conference". This also applies
when the endpoint has a connection in "loopback" or "continuity
test" mode. The details of this forwarding, e.g., RTP translator
or mixer, is outside the scope of this document.
Note that in order to detect events on a connection, the connection
must by default be in one of the modes "receive", "conference",
"send/receive", "network loopback" or "network continuity test". The
event detection only applies to the incoming media. Connections in
"sendonly", "inactive", "loopback", or "continuity test" mode will
thus normally not detect any events, although requesting to do so is
not considered an error.
The "loopback" and "continuity test" modes are used during
maintenance and continuity test operations. An endpoint may have
more than one connection in either "loopback" or "continuity test"
mode. As long as there is one connection in that particular mode,
and no other connection on the endpoint is placed in a different
maintenance or test mode, the maintenance or test operation shall
continue undisturbed. There are two flavors of continuity test, one
specified by ITU and one used in the US. In the first case, the test
is a loopback test. The originating switch will send a tone (the go
tone) on the bearer circuit and expects the terminating switch to
loopback the tone. If the originating switch sees the same tone
returned (the return tone), the COT has passed. If not, the COT has
failed. In the second case, the go and return tones are different.
The originating switch sends a certain go tone. The terminating
switch detects the go tone, it asserts a different return tone in the
backwards direction. When the originating switch detects the return
tone, the COT is passed. If the originating switch never detects the
return tone, the COT has failed.
If the mode is set to "loopback", the gateway is expected to return
the incoming signal from the endpoint back into that same endpoint.
This procedure will be used, typically, for testing the continuity of
trunk circuits according to the ITU specifications. If the mode is
set to "continuity test", the gateway is informed that the other end
of the circuit has initiated a continuity test procedure according to
the GR specification (see [22]). The gateway will place the circuit
in the transponder mode required for dual-tone continuity tests.
If the mode is set to "network loopback", the audio signals received
from the connection will be echoed back on the same connection. The
media is not forwarded to the endpoint.
If the mode is set to "network continuity test", the gateway will
process the packets received from the connection according to the
transponder mode required for dual-tone continuity test, and send the
processed signal back on the connection. The media is not forwarded
to the endpoint. The "network continuity test" mode is included for
backwards compatibility only and use of it is discouraged.
2.3.2 EndpointConfiguration
The EndpointConfiguration command can be used to specify the encoding
of the signals that will be received by the endpoint. For example,
in certain international telephony configurations, some calls will
carry mu-law encoded audio signals, while others will use A-law. The
Call Agent can use the EndpointConfiguration command to pass this
information to the gateway. The configuration may vary on a call by
call basis, but can also be used in the absence of any connection.
ReturnCode,
[PackageList]
<-- EndpointConfiguration(EndpointId,
[BearerInformation])
EndpointId is the name of the endpoint(s) in the gateway where
EndpointConfiguration executes. The "any of" wildcard convention
MUST NOT be used. If the "all of" wildcard convention is used, the
command applies to all the endpoints whose name matches the wildcard.
BearerInformation is a parameter defining the coding of the data sent
to and received from the line side. The information is encoded as a
list of sub-parameters. The only sub-parameter defined in this
version of the specification is the bearer encoding, whose value can
be set to "A-law" or "mu-law". The set of sub-parameters may be
extended.
In order to allow for extensibility, while remaining backwards
compatible, the BearerInformation parameter is conditionally optional
based on the following conditions:
* if Extension Parameters (vendor, package or other) are not used,
the BearerInformation parameter is REQUIRED,
* otherwise, the BearerInformation parameter is OPTIONAL.
When omitted, BearerInformation MUST retain its current value.
ReturnCode is a parameter returned by the gateway. It indicates the
outcome of the command and consists of an integer number optionally
followed by commentary.
PackageList is a list of supported packages that MAY be included with
error code 518 (unsupported package).
2.3.3 NotificationRequest
The NotificationRequest command is used to request the gateway to
send notifications upon the occurrence of specified events in an
endpoint. For example, a notification may be requested for when a
gateway detects that an endpoint is receiving tones associated with
fax communication. The entity receiving this notification may then
decide to specify use of a different type of encoding method in the
connections bound to this endpoint and instruct the gateway
accordingly with a ModifyConnection Command.
ReturnCode,
[PackageList]
<-- NotificationRequest(EndpointId,
[NotifiedEntity,]
[RequestedEvents,]
RequestIdentifier,
[DigitMap,]
[SignalRequests,]
[QuarantineHandling,]
[DetectEvents,]
[encapsulated EndpointConfiguration])
EndpointId is the identifier for the endpoint(s) in the the gateway
where the NotificationRequest executes. The "any of" wildcard MUST
NOT be used.
NotifiedEntity is an optional parameter that specifies a new
"notified entity" for the endpoint.
RequestIdentifier is used to correlate this request with the
notifications that it triggers. It will be repeated in the
corresponding Notify command.
RequestedEvents is a list of events, possibly qualified by event
parameters (see Section 3.2.2.4), that the gateway is requested to
detect and report. Such events may include, for example, fax tones,
continuity tones, or on-hook transition. Unless otherwise specified,
events are detected on the endpoint, however some events can be
detected on a connection. A given event MUST NOT appear more than
once in a RequestedEvents. If the parameter is omitted, it defaults
to empty.
To each event is associated one or more actions, which can be:
* Notify the event immediately, together with the accumulated list of
observed events,
* Swap audio,
* Accumulate the event in an event buffer, but don't notify yet,
* Accumulate according to Digit Map,
* Keep Signal(s) active,
* Process the Embedded Notification Request,
* Ignore the event.
Support for Notify, Accumulate, Keep Signal(s) Active, Embedded
Notification Request, and Ignore is REQUIRED. Support for Accumulate
according to Digit Map is REQUIRED on any endpoint capable of
detecting DTMF. Support for any other action is OPTIONAL. The set
of actions can be extended.
A given action can by default be specified for any event, although
some actions will not make sense for all events. For example, an
off-hook event with the Accumulate according to Digit Map action is
valid, but will of course immediately trigger a digit map mismatch
when the off-hook event occurs. Needless to say, such practice is
discouraged.
Some actions can be combined as shown in the table below, where "Y"
means the two actions can be combined, and "N" means they cannot:
--------------------------------------------------------------
Notif Swap Accum AccDi KeSiA EmbNo Ignor
--------------------------------------------------------------
Notif N Y N N Y Y* N
Swap - N Y N N N Y
Accum - - N N Y Y N
AccDi - - - N Y N N
KeSiA - - - - N Y Y
EmbNo - - - - - N N
Ignor - - - - - - N
--------------------------------------------------------------
Note (*): The "Embedded Notification Request" can only be
combined with "Notify", if the gateway is allowed to issue more
than one Notify command per Notification request (see below and
Section 4.4.1).
If no action is specified, the Notify action will be applied. If one
or more actions are specified, only those actions apply. When two or
more actions are specified, each action MUST be combinable with all
the other actions as defined by the table above - the individual
actions are assumed to occur simultaneously.
If a client receives a request with an invalid or unsupported action
or an illegal combination of actions, it MUST return an error to the
Call Agent (error code 523 - unknown or illegal combination of
actions, is RECOMMENDED).
In addition to the RequestedEvents parameter specified in the
command, some MGCP packages may contain "persistent events" (this is
generally discouraged though - see Appendix B for an alternative).
Persistent events in a given package are always detected on an
endpoint that implements that package. If a persistent event is not
included in the list of RequestedEvents, and the event occurs, the
event will be detected anyway and processed like all other events, as
if the persistent event had been requested with a Notify action. A
NotificationRequest MUST still be in place for a persistent event to
trigger a Notify though. Thus, informally, persistent events can be
viewed as always being implicitly included in the list of
RequestedEvents with an action to Notify, although no glare
detection, etc., will be performed.
Non-persistent events are those events that need to be explicitly
included in the RequestedEvents list. The (possibly empty) list of
requested events completely replaces the previous list of requested
events. In addition to the persistent events, only the events
specified in the requested events list will be detected by the
endpoint. If a persistent event is included in the RequestedEvents
list, the action specified will replace the default action associated
with the event for the life of the RequestedEvents list, after which
the default action is restored. For example, if "off-hook"was a
persistent event, the "Ignore off-hook" action was specified, and a
new request without any off-hook instructions were received, the
default "Notify off-hook" operation would be restored.
The gateway will detect the union of the persistent events and the
requested events. If an event is not included in either list, it
will be ignored.
The Call Agent can send a NotificationRequest with an empty (or
omitted) RequestedEvents list to the gateway. The Call Agent can do
so, for example, to a gateway when it does not want to collect any
more DTMF digits. However, persistent events will still be detected
and notified.
The Swap Audio action can be used when a gateway handles more than
one connection on an endpoint. This will be the case for call
waiting, and possibly other feature scenarios. In order to avoid the
round-trip to the Call Agent when just changing which connection is
attached to the audio functions of the endpoint, the
NotificationRequest can map an event (usually hook flash, but could
be some other event) to a local swap audio function, which selects
the "next" connection in a round robin fashion. If there is only one
connection, this action is effectively a no-op. If there are more
than two connections, the order is undefined. If the endpoint has
exactly two connections, one of which is "inactive", the other of
which is in "send/receive" mode, then swap audio will attempt to make
the "send/receive" connection "inactive", and vice versa. This
specification intentionally does not provide any additional detail on
the swap audio action.
If signal(s) are desired to start when an event being looked for
occurs, the "Embedded NotificationRequest" action can be used. The
embedded NotificationRequest may include a new list of
RequestedEvents, SignalRequests and a new digit map as well. The
semantics of the embedded NotificationRequest is as if a new
NotificationRequest was just received with the same NotifiedEntity,
RequestIdentifier, QuarantineHandling and DetectEvents. When the
"Embedded NotificationRequest" is activated, the "current dial
string" will be cleared; however the list of observed events and the
quarantine buffer will be unaffected (if combined with a Notify, the
Notify will clear the list of observed events though - see Section
4.4.1). Note, that the Embedded NotificationRequest action does not
accumulate the triggering event, however it can be combined with the
Accumulate action to achieve that. If the Embedded
NotificationRequest fails, an Embedded NotificationRequest failure
event SHOULD be generated (see Appendix B).
MGCP implementations SHALL be able to support at least one level of
embedding. An embedded NotificationRequest that respects this
limitation MUST NOT contain another Embedded NotificationRequest.
DigitMap is an optional parameter that allows the Call Agent to
provision the endpoint with a digit map according to which digits
will be accumulated. If this optional parameter is absent, the
previously defined value is retained. This parameter MUST be
defined, either explicitly or through a previous command, if the
RequestedEvents parameter contains a request to "accumulate according
to the digit map". The collection of these digits will result in a
digit string. The digit string is initialized to a null string upon
reception of the NotificationRequest, so that a subsequent
notification only returns the digits that were collected after this
request. Digits that were accumulated according to the digit map are
reported as any other accumulated event, in the order in which they
occur. It is therefore possible that other events accumulated are
found in between the list of digits. If the gateway is requested to
"accumulate according to digit map" and the gateway currently does
not have a digit map for the endpoint in question, the gateway MUST
return an error (error code 519 - endpoint does not have a digit map,
is RECOMMENDED).
SignalRequests is an optional parameter that contains the set of
signals that the gateway is asked to apply. When omitted, it
defaults to empty. When multiple signals are specified, the signals
MUST be applied in parallel. Unless otherwise specified, signals are
applied to the endpoint. However some signals can be applied to a
connection. Signals are identified by their name, which is an event
name, and may be qualified by signal parameters (see Section
3.2.2.4). The following are examples of signals:
* Ringing,
* Busy tone,
* Call waiting tone,
* Off hook warning tone,
* Ringback tones on a connection.
Names and descriptions of signals are defined in the appropriate
package.
Signals are, by default, applied to endpoints. If a signal applied
to an endpoint results in the generation of a media stream (audio,
video, etc.), then by default the media stream MUST NOT be forwarded
on any connection associated with that endpoint, regardless of the
mode of the connection. For example, if a call-waiting tone is
applied to an endpoint involved in an active call, only the party
using the endpoint in question will hear the call-waiting tone.
However, individual signals may define a different behavior.
When a signal is applied to a connection that has received a
RemoteConnectionDescriptor, the media stream generated by that signal
will be forwarded on the connection regardless of the current mode of
the connection (including loopback and continuity test). If a
RemoteConnectionDescriptor has not been received, the gateway MUST
return an error (error code 527 - missing RemoteConnectionDescriptor,
is RECOMMENDED). Note that this restriction does not apply to
detecting events on a connection.
When a (possibly empty) list of signal(s) is supplied, this list
completely replaces the current list of active time-out signals.
Currently active time-out signals that are not provided in the new
list MUST be stopped and the new signal(s) provided will now become
active. Currently active time-out signals that are provided in the
new list of signals MUST remain active without interruption, thus the
timer for such time-out signals will not be affected. Consequently,
there is currently no way to restart the timer for a currently active
time-out signal without turning the signal off first. If the time-
out signal is parameterized, the original set of parameters MUST
remain in effect, regardless of what values are provided
subsequently. A given signal MUST NOT appear more than once in a
SignalRequests. Note that applying a signal S to an endpoint,
connection C1 and connection C2, constitutes three different and
independent signals.
The action triggered by the SignalRequests is synchronized with the
collection of events specified in the RequestedEvents parameter. For
example, if the NotificationRequest mandates "ringing" and the
RequestedEvents asks to look for an "off-hook" event, the ringing
SHALL stop as soon as the gateway detects an off-hook event. The
formal definition is that the generation of all "Time Out" signals
SHALL stop as soon as one of the requested events is detected, unless
the "Keep signals active" action is associated to the detected event.
The RequestedEvents and SignalRequests may refer to the same event
definitions. In one case, the gateway is asked to detect the
occurrence of the event, and in the other case it is asked to
generate it. The specific events and signals that a given endpoint
can detect or perform are determined by the list of packages that are
supported by that endpoint. Each package specifies a list of events
and signals that can be detected or performed. A gateway that is
requested to detect or perform an event belonging to a package that
is not supported by the specified endpoint MUST return an error
(error code 518 - unsupported or unknown package, is RECOMMENDED).
When the event name is not qualified by a package name, the default
package name for the endpoint is assumed. If the event name is not
registered in this default package, the gateway MUST return an error
(error code 522 - no such event or signal, is RECOMMENDED).
The Call Agent can send a NotificationRequest whose requested signal
list is empty. It will do so for example when a time-out signal(s)
should stop.
If signal(s) are desired to start as soon as a "looked-for" event
occurs, the "Embedded NotificationRequest" action can be used. The
embedded NotificationRequest may include a new list of
RequestedEvents, SignalRequests and a new Digit Map as well. The
embedded NotificationRequest action allows the Call Agent to set up a
"mini-script" to be processed by the gateway immediately following
the detection of the associated event. Any SignalRequests specified
in the embedded NotificationRequest will start immediately.
Considerable care must be taken to prevent discrepancies between the
Call Agent and the gateway. However, long-term discrepancies should
not occur as a new SignalRequests completely replaces the old list of
active time-out signals, and BR-type signals always stop on their
own. Limiting the number of On/Off-type signals is encouraged. It
is considered good practice for a Call Agent to occasionally turn on
all On/Off signals that should be on, and turn off all On/Off signals
that should be off.
The Ignore action can be used to ignore an event, e.g., to prevent a
persistent event from being notified. However, the synchronization
between the event and an active time-out signal will still occur by
default (e.g., a time-out dial-tone signal will stop when an off-hook
occurs even if off-hook was a requested event with action "Ignore").
To prevent this synchronization from happening, the "Keep Signal(s)
Active" action will have to be specified as well.
The optional QuarantineHandling parameter specifies the handling of
"quarantine" events, i.e., events that have been detected by the
gateway before the arrival of this NotificationRequest command, but
have not yet been notified to the Call Agent. The parameter provides
a set of handling options (see Section 4.4.1 for details):
* whether the quarantined events should be processed or discarded
(the default is to process them).
* whether the gateway is expected to generate at most one
notification (step by step), or multiple notifications (loop), in
response to this request (the default is at most one).
When the parameter is absent, the default value is assumed.
We should note that the quarantine-handling parameter also governs
the handling of events that were detected and processed but not yet
notified when the command is received.
DetectEvents is an optional parameter, possibly qualified by event
parameters, that specifies a list of events that the gateway is
requested to detect during the quarantine period. When this
parameter is absent, the events to be detected in the quarantine
period are those listed in the last received DetectEvents list. In
addition, the gateway will also detect persistent events and the
events specified in the RequestedEvents list, including those for
which the "ignore" action is specified.
Some events and signals, such as the in-line ringback or the quality
alert, are performed or detected on connections terminating in the
endpoint rather than on the endpoint itself. The structure of the
event names (see Section 2.1.7) allows the Call Agent to specify the
connection(s) on which the events should be performed or detected.
The NotificationRequest command may carry an encapsulated
EndpointConfiguration command, that will apply to the same
endpoint(s). When this command is present, the parameters of the
EndpointConfiguration command are included with the normal parameters
of the NotificationRequest, with the exception of the EndpointId,
which is not replicated.
The encapsulated EndpointConfiguration command shares the fate of the
NotificationRequest command. If the NotificationRequest is rejected,
the EndpointConfiguration is not executed.
ReturnCode is a parameter returned by the gateway. It indicates the
outcome of the command and consists of an integer number optionally
followed by commentary.
PackageList is a list of supported packages that MAY be included with
error code 518 (unsupported package).
2.3.4 Notify
Notifications with the observed events are sent by the gateway via
the Notify command when a triggering event occurs.
ReturnCode,
[PackageList]
<-- Notify(EndpointId,
[NotifiedEntity,]
RequestIdentifier,
ObservedEvents)
EndpointId is the name for the endpoint in the gateway which is
issuing the Notify command. The identifier MUST be a fully qualified
endpoint identifier, including the domain name of the gateway. The
local part of the name MUST NOT use any of the wildcard conventions.
NotifiedEntity is a parameter that identifies the entity which
requested the notification. This parameter is equal to the
NotifiedEntity parameter of the NotificationRequest that triggered
this notification. The parameter is absent if there was no such
parameter in the triggering request. Regardless of the value of the
NotifiedEntity parameter, the notification MUST be sent to the
current "notified entity" for the endpoint.
RequestIdentifier is a parameter that repeats the RequestIdentifier
parameter of the NotificationRequest that triggered this
notification. It is used to correlate this notification with the
request that triggered it. Persistent events will be viewed here as
if they had been included in the last NotificationRequest. An
implicit NotificationRequest MAY be in place right after restart -
the RequestIdentifier used for it will be zero ("0") - see Section
4.4.1 for details.
ObservedEvents is a list of events that the gateway detected and
accumulated. A single notification may report a list of events that
will be reported in the order in which they were detected (FIFO).
The list will only contain the identification of events that were
requested in the RequestedEvents parameter of the triggering
NotificationRequest. It will contain the events that were either
accumulated (but not notified) or treated according to digit map (but
no match yet), and the final event that triggered the notification or
provided a final match in the digit map. It should be noted that
digits MUST be added to the list of observed events as they are
accumulated, irrespective of whether they are accumulated according
to the digit map or not. For example, if a user enters the digits
"1234" and some event E is accumulated between the digits "3" and "4"
being entered, the list of observed events would be "1, 2, 3, E, 4".
Events that were detected on a connection SHALL include the name of
that connection as in "R/qa@0A3F58" (see Section 2.1.7).
If the list of ObservedEvents reaches the capacity of the endpoint,
an ObservedEvents Full event (see Appendix B) SHOULD be generated
(the endpoint shall ensure it has capacity to include this event in
the list of ObservedEvents). If the ObservedEvents Full event is not
used to trigger a Notify, event processing continues as before
(including digit map matching); however, the subsequent events will
not be included in the list of ObservedEvents.
ReturnCode is a parameter returned by the Call Agent. It indicates
the outcome of the command and consists of an integer number
optionally followed by commentary.
PackageList is a list of supported packages that MAY be included with
error code 518 (unsupported package).
2.3.5 CreateConnection
This command is used to create a connection between two endpoints.
ReturnCode,
[ConnectionId,]
[SpecificEndPointId,]
[LocalConnectionDescriptor,]
[SecondEndPointId,]
[SecondConnectionId,]
[PackageList]
<-- CreateConnection(CallId,
EndpointId,
[NotifiedEntity,]
[LocalConnectionOptions,]
Mode,
[{RemoteConnectionDescriptor
SecondEndpointId}, ]
[Encapsulated NotificationRequest,]
[Encapsulated EndpointConfiguration])
A connection is defined by its endpoints. The input parameters in
CreateConnection provide the data necessary to build a gateway's
"view" of a connection.
CallId is a parameter that identifies the call (or session) to which
this connection belongs. This parameter SHOULD, at a minimum, be
unique within the collection of Call Agents that control the same
gateways. Connections that belong to the same call SHOULD share the
same call-id. The call-id has little semantic meaning in the
protocol; however it can be used to identify calls for reporting and
accounting purposes. It does not affect the handling of connections
by the gateway.
EndpointId is the identifier for the connection endpoint in the
gateway where CreateConnection executes. The EndpointId can be
fully-specified by assigning a value to the parameter EndpointId in
the function call or it may be under-specified by using the "any of"
wildcard convention. If the endpoint is underspecified, the endpoint
identifier SHALL be assigned by the gateway and its complete value
returned in the SpecificEndPointId parameter of the response. When
the "any of" wildcard is used, the endpoint assigned MUST be in-
service and MUST NOT already have any connections on it. If no such
endpoint is available, error code 410 (no endpoint available) SHOULD
be returned. The "all of" wildcard MUST NOT be used.
The NotifiedEntity is an optional parameter that specifies a new
"notified entity" for the endpoint.
LocalConnectionOptions is an optional structure used by the Call
Agent to direct the handling of the connection by the gateway. The
fields contained in a LocalConnectionOptions structure may include
one or more of the following (each field MUST NOT be supplied more
than once):
* Codec compression algorithm: One or more codecs, listed in order
of preference. For interoperability, it is RECOMMENDED to support
G.711 mu-law encoding ("PCMU"). See Section 2.6 for details on the
codec selection process.
* Packetization period: A single millisecond value or a range may be
specified. The packetization period SHOULD NOT contradict the
specification of the codec compression algorithm. If a codec is
specified that has a frame size which is inconsistent with the
packetization period, and that codec is selected, the gateway is
authorized to use a packetization period that is consistent with
the frame size even if it is different from that specified. In so
doing, the gateway SHOULD choose a non-zero packetization period as
close to that specified as possible. If a packetization period is
not specified, the endpoint SHOULD use the default packetization
period(s) for the codec(s) selected.
* Bandwidth: The allowable bandwidth, i.e., payload plus any header
overhead from the transport layer and up, e.g., IP, UDP, and RTP.
The bandwidth specification SHOULD NOT contradict the specification
of codec compression algorithm or packetization period. If a codec
is specified, then the gateway is authorized to use it, even if it
results in the usage of a larger bandwidth than specified. Any
discrepancy between the bandwidth and codec specification will not
be reported as an error.
* Type of Service: This indicates the class of service to be used
for this connection. When the Type of Service is not specified,
the gateway SHALL use a default value of zero unless provisioned
otherwise.
* Usage of echo cancellation: By default, the telephony gateways
always perform echo cancellation on the endpoint. However, it may
be necessary, for some calls, to turn off these operations. The
echo cancellation parameter can have two values, "on" (when the
echo cancellation is requested) and "off" (when it is turned off).
The parameter is optional. If the parameter is omitted when
creating a connection and there are no other connections on the
endpoint, the endpoint SHALL apply echo cancellation initially. If
the parameter is omitted when creating a connection and there are
existing connections on the endpoint, echo cancellation is
unchanged. The endpoint SHOULD subsequently enable or disable echo
cancellation when voiceband data is detected - see e.g., ITU-T
recommendation V.8, V.25, and G.168. Following termination of
voiceband data, the handling of echo cancellation SHALL then revert
to the current value of the echo cancellation parameter. It is
RECOMMENDED that echo cancellation handling is left to the gateway
rather than having this parameter specified by the Call Agent.
* Silence Suppression: The telephony gateways may perform voice
activity detection, and avoid sending packets during periods of
silence. However, it is necessary, for example for modem calls, to
turn off this detection. The silence suppression parameter can
have two values, "on" (when the detection is requested) and "off"
(when it is not requested). The default is "off" (unless
provisioned otherwise). Upon detecting voiceband data, the
endpoint SHOULD disable silence suppression. Following termination
of voiceband data, the handling of silence suppression SHALL then
revert to the current value of the silence suppression parameter.
* Gain Control: The telephony gateways may perform gain control on
the endpoint, in order to adapt the level of the signal. However,
it is necessary, for example for some modem calls, to turn off this
function. The gain control parameter may either be specified as
"automatic", or as an explicit number of decibels of gain. The
gain specified will be added to media sent out over the endpoint
(as opposed to the connection) and subtracted from media received
on the endpoint. The parameter is optional. When there are no
other connections on the endpoint, and the parameter is omitted,
the default is to not perform gain control (unless provisioned
otherwise), which is equivalent to specifying a gain of 0 decibels.
If there are other connections on the endpoint, and the parameter
is omitted, gain control is unchanged. Upon detecting voiceband
data, the endpoint SHOULD disable gain control if needed.
Following termination of voiceband data, the handling of gain
control SHALL then revert to the current value of the gain control
parameter. It should be noted, that handling of gain control is
normally best left to the gateway and hence use of this parameter
is NOT RECOMMENDED.
* RTP security: The Call agent can request the gateway to enable
encryption of the audio Packets. It does so by providing a key
specification, as specified in RFC2327. By default, encryption is
not performed.
* Network Type: The Call Agent may instruct the gateway to prepare
the connection on a specified type of network. If absent, the
value is based on the network type of the gateway being used.
* Resource reservation: The Call Agent may instruct the gateway to
use network resource reservation for the connection. See Section
2.7 for details.
The Call Agent specifies the relevant fields it cares about in the
command and leaves the rest to the discretion of the gateway. For
those of the above parameters that were not explicitly included, the
gateway SHOULD use the default values if possible. For a detailed
list of local connection options included with this specification
refer to section 3.2.2.10. The set of local connection options can
be extended.
The Mode indicates the mode of operation for this side of the
connection. The basic modes are "send", "receive", "send/receive",
"conference", "inactive", "loopback", "continuity test", "network
loop back" and "network continuity test". The expected handling of
these modes is specified in the introduction of the "Gateway Control
Commands", Section 2.3. Note that signals applied to a connection do
not follow the connection mode. Some endpoints may not be capable of
supporting all modes. If the command specifies a mode that the
endpoint does not support, an error SHALL be returned (error 517 -
unsupported mode, is RECOMMENDED). Also, if a connection has not yet
received a RemoteConnectionDescriptor, an error MUST be returned if
the connection is attempted to be placed in any of the modes "send
only", "send/receive", "conference", "network loopback", "network
continuity test", or if a signal (as opposed to detecting an event)
is to be applied to the connection (error code 527 - missing
RemoteConnectionDescriptor, is RECOMMENDED). The set of modes can be
extended.
The gateway returns a ConnectionId, that uniquely identifies the
connection within the endpoint, and a LocalConnectionDescriptor,
which is a session description that contains information about the
connection, e.g., IP address and port for the media, as defined in
SDP.
The SpecificEndPointId is an optional parameter that identifies the
responding endpoint. It is returned when the EndpointId argument
referred to an "any of" wildcard name and the command succeeded.
When a SpecificEndPointId is returned, the Call Agent SHALL use it as
the EndpointId value in successive commands referring to this
connection.
The SecondEndpointId can be used instead of the
RemoteConnectionDescriptor to establish a connection between two
endpoints located on the same gateway. The connection is by
definition a local connection. The SecondEndpointId can be fully-
specified by assigning a value to the parameter SecondEndpointId in
the function call or it may be under-specified by using the "any of"
wildcard convention. If the SecondEndpointId is underspecified, the
second endpoint identifier will be assigned by the gateway and its
complete value returned in the SecondEndPointId parameter of the
response.
When a SecondEndpointId is specified, the command really creates two
connections that can be manipulated separately through
ModifyConnection and DeleteConnection commands. In addition to the
ConnectionId and LocalConnectionDescriptor for the first connection,
the response to the creation provides a SecondConnectionId parameter
that identifies the second connection. The second connection is
established in "send/receive" mode.
After receiving a "CreateConnection" request that did not include a
RemoteConnectionDescriptor parameter, a gateway is in an ambiguous
situation. Because it has exported a LocalConnectionDescriptor
parameter, it can potentially receive packets. Because it has not
yet received the RemoteConnectionDescriptor parameter of the other
gateway, it does not know whether the packets that it receives have
been authorized by the Call Agent. It must thus navigate between two
risks, i.e., clipping some important announcements or listening to
insane data. The behavior of the gateway is determined by the value
of the Mode parameter:
* If the mode was set to ReceiveOnly, the gateway MUST accept the
media and transmit them through the endpoint.
* If the mode was set to Inactive, Loopback, or Continuity Test, the
gateway MUST NOT transmit the media through to the endpoint.
Note that the mode values SendReceive, Conference, SendOnly, Network
Loopback and Network Continuity Test do not make sense in this
situation. They MUST be treated as errors, and the command MUST be
rejected (error code 527 - missing RemoteConnectionDescriptor, is
RECOMMENDED).
The command may optionally contain an encapsulated Notification
Request command, which applies to the EndpointId, in which case a
RequestIdentifier parameter MUST be present, as well as, optionally,
other parameters of the NotificationRequest with the exception of the
EndpointId, which is not replicated. The encapsulated
NotificationRequest is executed simultaneously with the creation of
the connection. For example, when the Call Agent wants to initiate a
call to a residential gateway, it could:
* ask the residential gateway to prepare a connection, in order to be
sure that the user can start speaking as soon as the phone goes off
hook,
* ask the residential gateway to start ringing,
* ask the residential gateway to notify the Call Agent when the phone
goes off-hook.
This can be accomplished in a single CreateConnection command, by
also transmitting the RequestedEvents parameters for the off-hook
event, and the SignalRequests parameter for the ringing signal.
When these parameters are present, the creation and the
NotificationRequest MUST be synchronized, which means that both MUST
be accepted, or both MUST be refused. In our example, the
CreateConnection may be refused if the gateway does not have
sufficient resources, or cannot get adequate resources from the local
network access, and the off-hook NotificationRequest can be refused
in the glare condition, if the user is already off-hook. In this
example, the phone must not ring if the connection cannot be
established, and the connection must not be established if the user
is already off-hook.
The NotifiedEntity parameter, if present, defines the new "notified
entity" for the endpoint.
The command may carry an encapsulated EndpointConfiguration command,
which applies to the EndpointId. When this command is present, the
parameters of the EndpointConfiguration command are included with the
normal parameters of the CreateConnection with the exception of the
EndpointId, which is not replicated. The EndpointConfiguration
command may be encapsulated together with an encapsulated
NotificationRequest command. Note that both of these apply to the
EndpointId only.
The encapsulated EndpointConfiguration command shares the fate of the
CreateConnection command. If the CreateConnection is rejected, the
EndpointConfiguration is not executed.
ReturnCode is a parameter returned by the gateway. It indicates the
outcome of the command and consists of an integer number optionally
followed by commentary.
PackageList is a list of supported packages that MAY be included with
error code 518 (unsupported package).
2.3.6 ModifyConnection
This command is used to modify the characteristics of a gateway's
"view" of a connection. This "view" of the call includes both the
local connection descriptor as well as the remote connection
descriptor.
ReturnCode,
[LocalConnectionDescriptor,]
[PackageList]
<-- ModifyConnection(CallId,
EndpointId,
ConnectionId,
[NotifiedEntity,]
[LocalConnectionOptions,]
[Mode,]
[RemoteConnectionDescriptor,]
[Encapsulated NotificationRequest,]
[Encapsulated EndpointConfiguration])
The parameters used are the same as in the CreateConnection command,
with the addition of a ConnectionId that identifies the connection
within the endpoint. This parameter was returned by the
CreateConnection command, in addition to the local connection
descriptor. It uniquely identifies the connection within the context
of the endpoint. The CallId used when the connection was created
MUST be included as well.
The EndpointId MUST be a fully qualified endpoint identifier. The
local name MUST NOT use the wildcard conventions.
The ModifyConnection command can be used to affect parameters of a
connection in the following ways:
* Provide information about the other end of the connection, through
the RemoteConnectionDescriptor. If the parameter is omitted, it
retains its current value.
* Activate or deactivate the connection, by changing the value of the
Mode parameter. This can occur at any time during the connection,
with arbitrary parameter values. If the parameter is omitted, it
retains its current value.
* Change the parameters of the connection through the
LocalConnectionOptions, for example by switching to a different
coding scheme, changing the packetization period, or modifying the
handling of echo cancellation. If one or more
LocalConnectionOptions parameters are omitted, then the gateway
SHOULD refrain from changing that parameter from its current value,
unless another parameter necessitating such a change is explicitly
provided. For example, a codec change might require a change in
silence suppression. Note that if a RemoteConnectionDescriptor is
supplied, then only the LocalConnectionOptions actually supplied
with the ModifyConnection command will affect the codec negotiation
(as described in Section 2.6).
Connections can only be fully activated if the
RemoteConnectionDescriptor has been provided to the gateway. The
receive-only mode, however, can be activated without the provision of
this descriptor.
The command will only return a LocalConnectionDescriptor if the local
connection parameters, such as RTP ports, were modified. Thus, if,
for example, only the mode of the connection is changed, a
LocalConnectionDescriptor will not be returned. Note however, that
inclusion of LocalConnectionOptions in the command is not a
prerequisite for local connection parameter changes to occur. If a
connection parameter is omitted, e.g., silence suppression, the old
value of that parameter will be retained if possible. If a parameter
change necessitates a change in one or more unspecified parameters,
the gateway is free to choose suitable values for the unspecified
parameters that must change. This can for instance happen if the
packetization period was not specified. If the new codec supported
the old packetization period, the value of this parameter would not
change, as a change would not be necessary. However, if it did not
support the old packetization period, it would choose a suitable
value.
The command may optionally contain an encapsulated Notification
Request command, in which case a RequestIdentifier parameter MUST be
present, as well as, optionally, other parameters of the
NotificationRequest with the exception of the EndpointId, which is
not replicated. The encapsulated NotificationRequest is executed
simultaneously with the modification of the connection. For example,
when a connection is accepted, the calling gateway should be
instructed to place the circuit in send-receive mode and to stop
providing ringing tones. This can be accomplished in a single
ModifyConnection command, by also transmitting the RequestedEvents
parameters, for the on-hook event, and an empty SignalRequests
parameter, to stop the provision of ringing tones.
When these parameters are present, the modification and the
NotificationRequest MUST be synchronized, which means that both MUST
be accepted, or both MUST be refused.
The NotifiedEntity parameter, if present, defines the new "notified
entity" for the endpoint.
The command may carry an encapsulated EndpointConfiguration command,
that will apply to the same endpoint. When this command is present,
the parameters of the EndpointConfiguration command are included with
the normal parameters of the ModifyConnection with the exception of
the EndpointId, which is not replicated. The EndpointConfiguration
command may be encapsulated together with an encapsulated
NotificationRequest command.
The encapsulated EndpointConfiguration command shares the fate of the
ModifyConnection command. If the ModifyConnection is rejected, the
EndpointConfiguration is not executed.
ReturnCode is a parameter returned by the gateway. It indicates the
outcome of the command and consists of an integer number optionally
followed by commentary.
PackageList is a list of supported packages that MAY be included with
error code 518 (unsupported package).
2.3.7 DeleteConnection (from the Call Agent)
This command is used to terminate a connection. As a side effect, it
collects statistics on the execution of the connection.
ReturnCode,
ConnectionParameters,
[PackageList]
<-- DeleteConnection(CallId,
EndpointId,
ConnectionId,
[NotifiedEntity,]
[Encapsulated NotificationRequest,]
[Encapsulated EndpointConfiguration])
The endpoint identifier, in this form of the DeleteConnection
command, SHALL be fully qualified. Wildcard conventions SHALL NOT be
used.
The ConnectionId identifies the connection to be deleted. The CallId
used when the connection was created is included as well.
The NotifiedEntity parameter, if present, defines the new "notified
entity" for the endpoint.
In the case of IP multicast, connections can be deleted individually
and independently. However, in the unicast case where a connection
has two ends, a DeleteConnection command has to be sent to both
gateways involved in the connection. After the connection has been
deleted, media streams previously supported by the connection are no
longer available. Any media packets received for the old connection
are simply discarded and no new media packets for the stream are
sent.
After the connection has been deleted, any loopback that has been
requested for the connection must be cancelled (unless the endpoint
has another connection requesting loopback).
In response to the DeleteConnection command, the gateway returns a
list of connection parameters that describe statistics for the
connection.
When the connection was for an Internet media stream, these
parameters are:
Number of packets sent:
The total number of media packets transmitted by the sender since
starting transmission on this connection. In the case of RTP, the
count is not reset if the sender changes its synchronization
source identifier (SSRC, as defined in RTP), for example as a
result of a ModifyConnection command. The value is zero if the
connection was always set in "receive only" mode and no signals
were applied to the connection.
Number of octets sent:
The total number of payload octets (i.e., not including header or
padding) transmitted in media packets by the sender since starting
transmission on this connection. In the case of RTP, the count is
not reset if the sender changes its SSRC identifier, for example
as a result of a ModifyConnection command. The value is zero if
the connection was always set in "receive only" mode and no
signals were applied to the connection.
Number of packets received:
The total number of media packets received by the sender since
starting reception on this connection. In the case of RTP, the
count includes packets received from different SSRC, if the sender
used several values. The value is zero if the connection was
always set in "send only" mode.
Number of octets received:
The total number of payload octets (i.e., not including header,
e.g., RTP, or padding) transmitted in media packets by the sender
since starting transmission on this connection. In the case of
RTP, the count includes packets received from different SSRC, if
the sender used several values. The value is zero if the
connection was always set in "send only" mode.
Number of packets lost:
The total number of media packets that have been lost since the
beginning of reception. This number is defined to be the number
of packets expected less the number of packets actually received,
where the number of packets received includes any which are late
or duplicates. For RTP, the count includes packets received from
different SSRC, if the sender used several values. Thus packets
that arrive late are not counted as lost, and the loss may be
negative if there are duplicates. The count includes packets
received from different SSRC, if the sender used several values.
The number of packets expected is defined to be the extended last
sequence number received, as defined next, less the initial
sequence number received. The count includes packets received
from different SSRC, if the sender used several values. The value
is zero if the connection was always set in "send only" mode.
Interarrival jitter:
An estimate of the statistical variance of the media packet
interarrival time measured in milliseconds and expressed as an
unsigned integer. For RTP, the interarrival jitter J is defined
to be the mean deviation (smoothed absolute value) of the
difference D in packet spacing at the receiver compared to the
sender for a pair of packets. Detailed computation algorithms are
found in RFC1889. The count includes packets received from
different SSRC, if the sender used several values. The value is
zero if the connection was always set in "send only" mode.
Average transmission delay:
An estimate of the network latency, expressed in milliseconds. For
RTP, this is the average value of the difference between the NTP
timestamp indicated by the senders of the RTCP messages and the
NTP timestamp of the receivers, measured when the messages are
received. The average is obtained by summing all the estimates,
then dividing by the number of RTCP messages that have been
received. When the gateway's clock is not synchronized by NTP,
the latency value can be computed as one half of the round trip
delay, as measured through RTCP. When the gateway cannot compute
the one way delay or the round trip delay, the parameter conveys a
null value.
For a detailed definition of these variables, refer to RFC1889.
When the connection was set up over a LOCAL interconnect, the meaning
of these parameters is defined as follows:
Number of packets sent:
Not significant - MAY be omitted.
Number of octets sent:
The total number of payload octets transmitted over the local
connection.
Number of packets received:
Not significant - MAY be omitted.
Number of octets received:
The total number of payload octets received over the connection.
Number of packets lost:
Not significant - MAY be omitted. A value of zero is assumed.
Interarrival jitter:
Not significant - MAY be omitted. A value of zero is assumed.
Average transmission delay:
Not significant - MAY be omitted. A value of zero is assumed.
The set of connection parameters can be extended. Also, the meaning
may be further defined by other types of networks which MAY
furthermore elect to not return all, or even any, of the above
specified parameters.
The command may optionally contain an encapsulated Notification
Request command, in which case a RequestIdentifier parameter MUST be
present, as well as, optionally, other parameters of the
NotificationRequest with the exception of the EndpointId, which is
not replicated. The encapsulated NotificationRequest is executed
simultaneously with the deletion of the connection. For example,
when a user hang-up is notified, the gateway should be instructed to
delete the connection and to start looking for an off-hook event.
This can be accomplished in a single DeleteConnection command, by
also transmitting the RequestedEvents parameters, for the off-hook
event, and an empty SignalRequests parameter.
When these parameters are present, the DeleteConnection and the
NotificationRequest must be synchronized, which means that both MUST
be accepted, or both MUST be refused.
The command may carry an encapsulated EndpointConfiguration command,
that will apply to the same endpoint. When this command is present,
the parameters of the EndpointConfiguration command are included with
the normal parameters of the DeleteConnection with the exception of
the EndpointId, which is not replicated. The EndpointConfiguration
command may be encapsulated together with an encapsulated
NotificationRequest command.
The encapsulated EndpointConfiguration command shares the fate of the
DeleteConnection command. If the DeleteConnection is rejected, the
EndpointConfiguration is not executed.
ReturnCode is a parameter returned by the gateway. It indicates the
outcome of the command and consists of an integer number optionally
followed by commentary.
PackageList is a list of supported packages that MAY be included with
error code 518 (unsupported package).
2.3.8 DeleteConnection (from the gateway)
In some rare circumstances, a gateway may have to clear a connection,
for example because it has lost the resource associated with the
connection, or because it has detected that the endpoint no longer is
capable or willing to send or receive media. The gateway may then
terminate the connection by using a variant of the DeleteConnection
command:
ReturnCode,
[PackageList]
<-- DeleteConnection(CallId,
EndpointId,
ConnectionId,
ReasonCode,
Connection-parameters)
The EndpointId, in this form of the DeleteConnection command, MUST be
fully qualified. Wildcard conventions MUST NOT be used.
The ReasonCode is a text string starting with a numeric reason code
and optionally followed by a descriptive text string. The reason
code indicates the cause of the DeleteConnection. A list of reason
codes can be found in Section 2.5.
In addition to the call, endpoint and connection identifiers, the
gateway will also send the connection parameters that would have been
returned to the Call Agent in response to a DeleteConnection command.
ReturnCode is a parameter returned by the Call Agent. It indicates
the outcome of the command and consists of an integer number
optionally followed by commentary.
PackageList is a list of supported packages that MAY be included with
error code 518 (unsupported package).
Note that use of this command is generally discouraged and should
only be done as a last resort. If a connection can be sustained,
deletion of it should be left to the discretion of the Call Agent
which is in a far better position to make intelligent decisions in
this area.
2.3.9 DeleteConnection (multiple connections from the Call Agent)
A variation of the DeleteConnection function can be used by the Call
Agent to delete multiple connections at the same time. Note that
encapsulating other commands with this variation of the
DeleteConnection command is not permitted. The command can be used
to delete all connections that relate to a Call for an endpoint:
ReturnCode,
[PackageList]
<-- DeleteConnection(CallId,
EndpointId)
The EndpointId, in this form of the DeleteConnection command, MUST
NOT use the "any of" wildcard. All connections for the endpoint(s)
with the CallId specified will be deleted. Note that the command
will still succeed if there were no connections with the CallId
specified, as long as the EndpointId was valid. However, if the
EndpointId is invalid, the command will fail. The command does not
return any individual statistics or call parameters.
It can also be used to delete all connections that terminate in a
given endpoint:
ReturnCode,
[PackageList]
<-- DeleteConnection(EndpointId)
The EndpointId, in this form of the DeleteConnection command, MUST
NOT use the "any of" wildcard. Again, the command succeeds even if
there were no connections on the endpoint(s).
Finally, Call Agents can take advantage of the hierarchical structure
of endpoint names to delete all the connections that belong to a
group of endpoints. In this case, the "local name" component of the
EndpointId will be specified using the "all of" wildcarding
convention. The "any of" convention SHALL NOT be used. For example,
if endpoint names are structured as the combination of a physical
interface name and a circuit number, as in "X35V3+A4/13", the Call
Agent may replace the circuit number by the "all of" wild card
character "*", as in "X35V3+A4/*". This "wildcard" command instructs
the gateway to delete all the connections that were attached to
circuits connected to the physical interface "X35V3+A4".
After all the connections have been deleted, any loopback that has
been requested for the connections MUST be cancelled by the gateway.
This command does not return any individual statistics or call
parameters.
ReturnCode is a parameter returned by the gateway. It indicates the
outcome of the command and consists of an integer number optionally
followed by commentary.
PackageList is a list of supported packages that MAY be included with
error code 518 (unsupported package).
2.3.10 AuditEndpoint
The AuditEndPoint command can be used by the Call Agent to find out
the status of a given endpoint.
ReturnCode,
EndPointIdList,{
[RequestedEvents,]
[QuarantineHandling,]
[DigitMap,]
[SignalRequests,]
[RequestIdentifier,]
[NotifiedEntity,]
[ConnectionIdentifiers,]
[DetectEvents,]
[ObservedEvents,]
[EventStates,]
[BearerInformation,]
[RestartMethod,]
[RestartDelay,]
[ReasonCode,]
[MaxMGCPDatagram,]
[Capabilities]}
[PackageList]
<-- AuditEndPoint(EndpointId,
[RequestedInfo])
The EndpointId identifies the endpoint(s) being audited. The "any
of" wildcard convention MUST NOT be used.
The EndpointId identifies the endpoint(s) being audited. The "all
of" wildcard convention can be used to start auditing of a group of
endpoints (regardless of their service-state). If this convention is
used, the gateway SHALL return the list of endpoint identifiers that
match the wildcard in the EndPointIdList parameter, which is simply
one or more SpecificEndpointIds (each supplied separately). In the
case where the "all of" wildcard is used, RequestedInfo SHOULD NOT be
included (if it is included, it MUST be ignored). Note that the use
of the "all of" wildcard can potentially generate a large
EndPointIdList. If the resulting EndPointIdList is considered too
large, the gateway returns an error (error code 533 - response too
large, is RECOMMENDED).
When a non-wildcard EndpointId is specified, the (possibly empty)
RequestedInfo parameter describes the information that is requested
for the EndpointId specified. The following endpoint info can be
audited with this command:
RequestedEvents, DigitMap, SignalRequests, RequestIdentifier,
QuarantineHandling, NotifiedEntity, ConnectionIdentifiers,
DetectEvents, ObservedEvents, EventStates, BearerInformation,
RestartMethod, RestartDelay, ReasonCode, PackageList,
MaxMGCPDatagram, and Capabilities.
The list may be extended by extension parameters. The response will
in turn include information about each of the items for which
auditing info was requested. Supported parameters with empty values
MUST always be returned. However, if an endpoint is queried about a
parameter it does not understand, the endpoint MUST NOT generate an
error; instead the parameter MUST be omitted from the response:
* RequestedEvents: The current value of RequestedEvents the endpoint
is using including the action(s) and event parameters associated
with each event - if no actions are included, the default action is
assumed. Persistent events are included in the list. If an embedded
NotificationRequest is active, the RequestedEvents will reflect the
events requested in the embedded NotificationRequest, not any
surrounding RequestedEvents (whether embedded or not).
* DigitMap: The digit map the endpoint is currently using. The
parameter will be empty if the endpoint does not have a digit map.
* SignalRequests: A list of the; Time-Out signals that are currently
active, On/Off signals that are currently "on" for the endpoint
(with or without parameter), and any pending Brief signals. Time-
Out signals that have timed-out, and currently playing Brief
signals are not included. Any signal parameters included in the
original SignalRequests will be included.
* RequestIdentifier: The RequestIdentifier for the last
NotificationRequest received by this endpoint (includes
NotificationRequests encapsulated in other commands). If no
NotificationRequest has been received since reboot/restart, the
value zero will be returned.
* QuarantineHandling: The QuarantineHandling for the last
NotificationRequest received by this endpoint. If
QuarantineHandling was not included, or no notification request has
been received, the default values will be returned.
* DetectEvents: The value of the most recently received DetectEvents
parameter plus any persistent events implemented by the endpoint.
If no DetectEvents parameter has been received, the (possibly
empty) list only includes persistent events.
* NotifiedEntity: The current "notified entity" for the endpoint.
* ConnectionIdentifiers: The list of ConnectionIdentifiers for all
connections that currently exist for the specified endpoint.
* ObservedEvents: The current list of observed events for the
endpoint.
* EventStates: For events that have auditable states associated with
them, the event corresponding to the state the endpoint is in,
e.g., off-hook if the endpoint is off-hook. Note that the
definition of the individual events will state if the event in
question has an auditable state associated with it.
* BearerInformation: The value of the last received
BearerInformation parameter for this endpoint (this includes the
case where BearerInformation was provisioned). The parameter will
be empty if the endpoint has not received a BearerInformation
parameter and a value was also not provisioned.
* RestartMethod: "restart" if the endpoint is in-service and
operation is normal, or if the endpoint is in the process of
becoming in-service (a non-zero RestartDelay will indicate the
latter). Otherwise, the value of the restart method parameter in
the last RestartInProgress command issued (or should have been
issued) by the endpoint. Note that a "disconnected" endpoint will
thus only report "disconnected" as long as it actually is
disconnected, and "restart" will be reported once it is no longer
disconnected. Similarly, "cancel-graceful" will not be reported,
but "graceful" might (see Section 4.4.5 for further details).
* RestartDelay: The value of the restart delay parameter if a
RestartInProgress command was to be issued by the endpoint at the
time of this response, or zero if the command would not include
this parameter.
* ReasonCode: The value of the ReasonCode parameter in the last
RestartInProgress or DeleteConnection command issued by the gateway
for the endpoint, or the special value 000 if the endpoint's state
is normal.
* PackageList: The packages supported by the endpoint including
package version numbers. For backwards compatibility, support for
the parameter is OPTIONAL although implementations with package
versions higher than zero SHOULD support it.
* MaxMGCPDatagram: The maximum size of an MGCP datagram in bytes
that can be received by the endpoint (see Section 3.5.4). The
value excludes any lower layer overhead. For backwards
compatibility, support for this parameter is OPTIONAL. The default
maximum MGCP datagram size SHOULD be assumed if a value is not
returned.
* Capabilities: The capabilities for the endpoint similar to the
LocalConnectionOptions parameter and including packages and
connection modes. Extensions MAY be included as well. If any
unknown capabilities are reported, they MUST simply be ignored. If
there is a need to specify that some parameters, such as e.g.,
silence suppression, are only compatible with some codecs, then the
gateway MUST return several capability sets, each of which may
include:
- Compression Algorithm: A list of supported codecs. The rest of
the parameters in the capability set will apply to all codecs
specified in this list.
- Packetization Period: A single value or a range may be
specified.
- Bandwidth: A single value or a range corresponding to the range
for packetization periods may be specified (assuming no silence
suppression).
- Echo Cancellation: Whether echo cancellation is supported or not
for the endpoint.
- Silence Suppression: Whether silence suppression is supported or
not.
- Gain Control: Whether gain control is supported or not.
- Type of Service: Whether type of service is supported or not.
- Resource Reservation: Whether resource reservation is supported
or not.
- Security: Whether media encryption is supported or not.
- Type of network: The type(s) of network supported.
- Packages: A list of packages supported. The first package in
the list will be the default package.
- Modes: A list of supported connection modes.
The Call Agent may then decide to use the AuditConnection command to
obtain further information about the connections.
If no info was requested and the EndpointId refers to a valid
endpoint (in-service or not), the gateway simply returns a positive
acknowledgement.
ReturnCode is a parameter returned by the gateway. It indicates the
outcome of the command and consists of an integer number optionally
followed by commentary.
Note that PackageList MAY also be included with error code 518
(unsupported package).
2.3.11 AuditConnection
The AuditConnection command can be used by the Call Agent to retrieve
the parameters attached to a connection.
ReturnCode,
[CallId,]
[NotifiedEntity,]
[LocalConnectionOptions,]
[Mode,]
[RemoteConnectionDescriptor,]
[LocalConnectionDescriptor,]
[ConnectionParameters,]
[PackageList]
<-- AuditConnection(EndpointId,
ConnectionId,
RequestedInfo)
The EndpointId parameter specifies the endpoint that handles the
connection. The wildcard conventions SHALL NOT be used.
The ConnectionId parameter is the identifier of the audited
connection, within the context of the specified endpoint.
The (possibly empty) RequestedInfo describes the information that is
requested for the ConnectionId within the EndpointId specified. The
following connection info can be audited with this command:
CallId, NotifiedEntity, LocalConnectionOptions, Mode,
RemoteConnectionDescriptor, LocalConnectionDescriptor,
ConnectionParameters
The AuditConnection response will in turn include information about
each of the items auditing info was requested for:
* CallId, the CallId for the call the connection belongs to.
* NotifiedEntity, the current "notified entity" for the Connection.
Note this is the same as the "notified entity" for the endpoint
(included here for backwards compatibility).
* LocalConnectionOptions, the most recent LocalConnectionOptions
parameters that was actually supplied for the connection (omitting
LocalConnectionOptions from a command thus does not change this
value). Note that default parameters omitted from the most recent
LocalConnectionOptions will not be included.
LocalConnectionOptions that retain their value across
ModifyConnection commands and which have been included in a
previous command for the connection are also included, regardless
of whether they were supplied in the most recent
LocalConnectionOptions or not.
* Mode, the current mode of the connection.
* RemoteConnectionDescriptor, the RemoteConnectionDescriptor that was
supplied to the gateway for the connection.
* LocalConnectionDescriptor, the LocalConnectionDescriptor the
gateway supplied for the connection.
* ConnectionParameters, the current values of the connection
parameters for the connection.
If no info was requested and the EndpointId is valid, the gateway
simply checks that the connection exists, and if so returns a
positive acknowledgement. Note, that by definition, the endpoint
must be in-service for this to happen, as out-of-service endpoints do
not have any connections.
ReturnCode is a parameter returned by the gateway. It indicates the
outcome of the command and consists of an integer number optionally
followed by commentary.
PackageList is a list of supported packages that MAY be included with
error code 518 (unsupported package).
2.3.12 RestartInProgress
The RestartInProgress command is used by the gateway to signal that
an endpoint, or a group of endpoints, is put in-service or out-of-
service.
ReturnCode,
[NotifiedEntity,]
[PackageList]
<-- RestartInProgress(EndPointId,
RestartMethod,
[RestartDelay,]
[ReasonCode])
The EndPointId identifies the endpoint(s) that are put in-service or
out-of-service. The "all of" wildcard convention may be used to
apply the command to a group of endpoints managed by the same Call
Agent, such as for example all endpoints that are attached to a
specified interface, or even all endpoints that are attached to a
given gateway. The "any of" wildcard convention SHALL NOT be used.
The RestartMethod parameter specifies the type of restart. The
following values have been defined:
* A "graceful" restart method indicates that the specified endpoints
will be taken out-of-service after the specified delay. The
established connections are not yet affected, but the Call Agent
SHOULD refrain from establishing new connections, and SHOULD try to
gracefully tear down the existing connections.
* A "forced" restart method indicates that the specified endpoints
are taken abruptly out-of-service. The established connections, if
any, are lost.
* A "restart" method indicates that service will be restored on the
endpoints after the specified "restart delay", i.e., the endpoints
will be in-service. The endpoints are in their clean default state
and there are no connections that are currently established on the
endpoints.
* A "disconnected" method indicates that the endpoint has become
disconnected and is now trying to establish connectivity (see
Section 4.4.7). The "restart delay" specifies the number of
seconds the endpoint has been disconnected. Established
connections are not affected.
* A "cancel-graceful" method indicates that a gateway is canceling a
previously issued "graceful" restart command. The endpoints are
still in-service.
The list of restart methods may be extended.
The optional "restart delay" parameter is expressed as a number of
seconds. If the number is absent, the delay value MUST be considered
null (i.e., zero). In the case of the "graceful" method, a null
delay indicates that the Call Agent SHOULD simply wait for the
natural termination of the existing connections, without establishing
new connections. The restart delay is always considered null in the
case of the "forced" and "cancel-graceful" methods, and hence the
"restart delay" parameter MUST NOT be used with these restart
methods. When the gateway sends a "restart" or "graceful"
RestartInProgress message with a non-zero restart delay, the gateway
SHOULD send an updated RestartInProgress message after the "restart
delay" has passed.
A restart delay of null for the "restart" method indicates that
service has already been restored. This typically will occur after
gateway startup/reboot. To mitigate the effects of a gateway IP
address change as a result of a re-boot, the Call Agent MAY wish to
either flush its DNS cache for the gateway's domain name or resolve
the gateway's domain name by querying the DNS regardless of the TTL
of a current DNS resource record for the restarted gateway.
The optional reason code parameter indicates the cause of the
restart.
Gateways SHOULD send a "graceful" or "forced" RestartInProgress
message (for the relevant endpoints) as a courtesy to the Call Agent
when they are taken out-of-service, e.g., by being shutdown, or taken
out-of-service by a network management system, however the Call Agent
cannot rely on always receiving such a message. Gateways MUST send a
"restart" RestartInProgress message (for the relevant endpoints) with
a null delay to their Call Agent when they are back in-service
according to the restart procedure specified in Section 4.4.6 - Call
Agents can rely on receiving this message. Also, gateways MUST send
a "disconnected" RestartInProgress message (for the relevant
endpoints) to their current "notified entity" according to the
"disconnected" procedure specified in Section 4.4.7.
The RestartInProgress message will be sent to the current "notified
entity" for the EndpointId in question. It is expected that a
default Call Agent, i.e., "notified entity", has been provisioned so
that after a reboot/restart, the default Call Agent will always be
the "notified entity" for the endpoint. Gateways SHOULD take full
advantage of wild-carding to minimize the number of RestartInProgress
messages generated when multiple endpoints in a gateway restart and
the endpoints are managed by the same Call Agent.
ReturnCode is a parameter returned by the Call Agent. It indicates
the outcome of the command and consists of an integer number
optionally followed by commentary.
A NotifiedEntity may additionally be returned with the response to
the RestartInProgress from the Call Agent - this SHOULD normally only
be done in response to "restart" or "disconnected" (see also Section
4.4.6 and 4.4.7):
* If the response indicated success (return code 200 - transaction
executed), the restart in question completed successfully, and the
NotifiedEntity returned is the new "notified entity" for the
endpoint(s).
* If the response from the Call Agent indicated an error, the restart
in question did not complete successfully. If a NotifiedEntity
parameter was included in the response returned, it specifies a new
"notified entity" for the endpoint(s), which MUST be used when
retrying the restart in question (as a new transaction). This
SHOULD only be done with error code 521 (endpoint redirected).
Note that the above behavior for returning a NotifiedEntity in the
response is only defined for RestartInProgress responses and SHOULD
NOT be done for responses to other commands. Any other behavior is
undefined.
PackageList is a list of supported packages that MAY be included with
error code 518 (unsupported package).
2.4 Return Codes and Error Codes
All MGCP commands are acknowledged. The acknowledgment carries a
return code, which indicates the status of the command. The return
code is an integer number, for which the following ranges of values
have been defined:
* values between 000 and 099 indicate a response acknowledgement
* values between 100 and 199 indicate a provisional response
* values between 200 and 299 indicate a successful completion
* values between 400 and 499 indicate a transient error
* values between 500 and 599 indicate a permanent error
* values between 800 and 899 are package specific response codes.
A broad description of transient errors (4XX error codes) versus
permanent errors (5XX error codes) is as follows:
* If a Call Agent receives a transient error, there is the
expectation of the possibility that a future similar request will
be honored by the endpoint. In some cases, this may require some
state change in the environment of the endpoint (e.g., hook state
as in the case of error codes 401 or 402; resource availability as
in the case of error code 403, or bandwidth availability as in the
case of error code 404).
* Permanent errors (error codes 500 to 599) indicate one or more
permanent conditions either due to protocol error or
incompatibility between the endpoint and the Call Agent, or because
of some error condition over which the Call Agent has no control.
Examples are protocol errors, requests for endpoint capabilities
that do not exist, errors on interfaces associated with the
endpoint, missing or incorrect information in the request or any
number of other conditions which will simply not disappear with
time.
The values that have been already defined are the following:
000 Response Acknowledgement.
100 The transaction is currently being executed. An actual
completion message will follow later.
101 The transaction has been queued for execution. An actual
completion message will follow later.
200 The requested transaction was executed normally. This return
code can be used for a successful response to any command.
250 The connection was deleted. This return code can only be used
for a successful response to a DeleteConnection command.
400 The transaction could not be executed, due to some unspecified
transient error.
401 The phone is already off hook.
402 The phone is already on hook.
403 The transaction could not be executed, because the endpoint does
not have sufficient resources at this time.
404 Insufficient bandwidth at this time.
405 The transaction could not be executed, because the endpoint is
"restarting".
406 Transaction time-out. The transaction did not complete in a
reasonable period of time and has been aborted.
407 Transaction aborted. The transaction was aborted by some
external action, e.g., a ModifyConnection command aborted by a
DeleteConnection command.
409 The transaction could not be executed because of internal
overload.
410 No endpoint available. A valid "any of" wildcard was used,
however there was no endpoint available to satisfy the request.
500 The transaction could not be executed, because the endpoint is
unknown.
501 The transaction could not be executed, because the endpoint is
not ready. This includes the case where the endpoint is out-of-
service.
502 The transaction could not be executed, because the endpoint does
not have sufficient resources (permanent condition).
503 "All of" wildcard too complicated.
504 Unknown or unsupported command.
505 Unsupported RemoteConnectionDescriptor. This SHOULD be used when
one or more mandatory parameters or values in the
RemoteConnectionDescriptor is not supported.
506 Unable to satisfy both LocalConnectionOptions and
RemoteConnectionDescriptor. This SHOULD be used when the
LocalConnectionOptions and RemoteConnectionDescriptor contain one
or more mandatory parameters or values that conflict with each
other and/or cannot be supported at the same time (except for
codec negotiation failure - see error code 534).
507 Unsupported functionality. Some unspecified functionality
required to carry out the command is not supported. Note that
several other error codes have been defined for specific areas of
unsupported functionality (e.g. 508, 511, etc.), and this error
code SHOULD only be used if there is no other more specific error
code for the unsupported functionality.
508 Unknown or unsupported quarantine handling.
509 Error in RemoteConnectionDescriptor. This SHOULD be used when
there is a syntax or semantic error in the
RemoteConnectionDescriptor.
510 The transaction could not be executed, because some unspecified
protocol error was detected. Automatic recovery from such an
error will be very difficult, and hence this code SHOULD only be
used as a last resort.
511 The transaction could not be executed, because the command
contained an unrecognized extension. This code SHOULD be used
for unsupported critical parameter extensions ("X+").
512 The transaction could not be executed, because the gateway is not
equipped to detect one of the requested events.
513 The transaction could not be executed, because the gateway is not
equipped to generate one of the requested signals.
514 The transaction could not be executed, because the gateway cannot
send the specified announcement.
515 The transaction refers to an incorrect connection-id (may have
been already deleted).
516 The transaction refers to an unknown call-id, or the call-id
supplied is incorrect (e.g., connection-id not associated with
this call-id).
517 Unsupported or invalid mode.
518 Unsupported or unknown package. It is RECOMMENDED to include a
PackageList parameter with the list of supported packages in the
response, especially if the response is generated by the Call
Agent.
519 Endpoint does not have a digit map.
520 The transaction could not be executed, because the endpoint is
"restarting". In most cases this would be a transient error, in
which case, error code 405 SHOULD be used instead. The error
code is only included here for backwards compatibility.
521 Endpoint redirected to another Call Agent. The associated
redirection behavior is only well-defined when this response is
issued for a RestartInProgress command.
522 No such event or signal. The request referred to an event or
signal that is not defined in the relevant package (which could
be the default package).
523 Unknown action or illegal combination of actions.
524 Internal inconsistency in LocalConnectionOptions.
525 Unknown extension in LocalConnectionOptions. This code SHOULD be
used for unsupported mandatory vendor extensions ("x+").
526 Insufficient bandwidth. In cases where this is a transient
error, error code 404 SHOULD be used instead.
527 Missing RemoteConnectionDescriptor.
528 Incompatible protocol version.
529 Internal hardware failure.
530 CAS signaling protocol error.
531 Failure of a grouping of trunks (e.g., facility failure).
532 Unsupported value(s) in LocalConnectionOptions.
533 Response too large.
534 Codec negotiation failure.
535 Packetization period not supported.
536 Unknown or unsupported RestartMethod.
537 Unknown or unsupported digit map extension.
538 Event/signal parameter error (e.g., missing, erroneous,
unsupported, unknown, etc.).
539 Invalid or unsupported command parameter. This code SHOULD only
be used when the parameter is neither a package or vendor
extension parameter.
540 Per endpoint connection limit exceeded.
541 Invalid or unsupported LocalConnectionOptions. This code SHOULD
only be used when the LocalConnectionOptions is neither a package
nor a vendor extension LocalConnectionOptions.
The set of return codes may be extended in a future version of the
protocol. Implementations that receive an unknown or unsupported
return code SHOULD treat the return code as follows:
* Unknown 0xx code treated as 000.
* Unknown 1xx code treated as 100.
* Unknown 2xx code treated as 200.
* Unknown 3xx code treated as 521.
* Unknown 4xx code treated as 400.
* Unknown 5xx-9xx code treated as 510.
2.5 Reason Codes
Reason codes are used by the gateway when deleting a connection to
inform the Call Agent about the reason for deleting the connection.
They may also be used in a RestartInProgress command to inform the
Call Agent of the reason for the RestartInProgress.
The reason code is an integer number, and the following values have
been defined:
000 Endpoint state is normal (this code is only used in response to
audit requests).
900 Endpoint malfunctioning.
901 Endpoint taken out-of-service.
902 Loss of lower layer connectivity (e.g., downstream sync).
903 QoS resource reservation was lost.
904 Manual intervention.
905 Facility failure (e.g., DS-0 failure).
The set of reason codes can be extended.
2.6 Use of Local Connection Options and Connection Descriptors
As indicated previously, the normal sequence in setting up a bi-
directional connection involves at least 3 steps:
1) The Call Agent asks the first gateway to "create a connection" on
an endpoint. The gateway allocates resources to that connection,
and responds to the command by providing a "session description"
(referred to as its LocalConnectionDescriptor). The session
description contains the information necessary for another party
to send packets towards the newly created connection.
2) The Call Agent then asks the second gateway to "create a
connection" on an endpoint. The command carries the "session
description" provided by the first gateway (now referred to as the
RemoteConnectionDescriptor). The gateway allocates resources to
that connection, and responds to the command by providing its own
"session description" (LocalConnectionDescriptor).
3) The Call Agent uses a "modify connection" command to provide this
second "session description" (now referred to as the
RemoteConnectionDescriptor ) to the first endpoint. Once this is
done, communication can proceed in both directions.
When the Call Agent issues a Create or Modify Connection command,
there are thus three parameters that determine the media supported by
that connection:
* LocalConnectionOptions: Supplied by the Call Agent to control the
media parameters used by the gateway for the connection. When
supplied, the gateway MUST conform to these media parameters until
either the connection is deleted, or a ModifyConnection command
with new media parameters (LocalConnectionOptions or
RemoteConnectionDescriptor) is received.
* RemoteConnectionDescriptor: Supplied by the Call Agent to convey
the media parameters supported by the other side of the connection.
When supplied, the gateway MUST conform to these media parameters
until either the connection is deleted, or a ModifyConnection
command with new media parameters (LocalConnectionOptions or
RemoteConnectionDescriptor) is received.
* LocalConnectionDescriptor: Supplied by the gateway to the Call
Agent to convey the media parameters it supports for the
connection. When supplied, the gateway MUST honor the media
parameters until either the connection is deleted, or the gateway
issues a new LocalConnectionDescriptor for that connection.
In determining which codec(s) to provide in the
LocalConnectionDescriptor, there are three lists of codecs that a
gateway needs to consider:
* A list of codecs allowed by the LocalConnectionOptions in the
current command (either explicitly by encoding method or implicitly
by bandwidth and/or packetization period).
* A list of codecs in the RemoteConnectionDescriptor in the current
command.
* An internal list of codecs that the gateway can support for the
connection. A gateway MAY support one or more codecs for a given
connection.
Codec selection (including all relevant media parameters) can then be
described by the following steps:
1. An approved list of codecs is formed by taking the intersection of
the internal list of codecs and codecs allowed by the
LocalConnectionOptions. If LocalConnectionOptions were not
provided in the current command, the approved list of codecs thus
contains the internal list of codecs.
2. If the approved list of codecs is empty, a codec negotiation
failure has occurred and an error response is generated (error
code 534 - codec negotiation failure, is RECOMMENDED).
3. Otherwise, a negotiated list of codecs is formed by taking the
intersection of the approved list of codecs and codecs allowed by
the RemoteConnectionDescriptor. If a RemoteConnectionDescriptor
was not provided in the current command, the negotiated list of
codecs thus contains the approved list of codecs.
4. If the negotiated list of codecs is empty, a codec negotiation
failure has occurred and an error response is generated (error
code 534 - codec negotiation failure, is RECOMMENDED).
5. Otherwise, codec negotiation has succeeded, and the negotiated
list of codecs is returned in the LocalConnectionDescriptor.
Note that both LocalConnectionOptions and the
RemoteConnectionDescriptor can contain a list of codecs ordered by
preference. When both are supplied in the current command, the
gateway MUST adhere to the preferences provided in the
LocalConnectionOptions.
2.7 Resource Reservations
The gateways can be instructed to perform a reservation, for example
using RSVP, on a given connection. When a reservation is needed, the
call agent will specify the reservation profile to be used, which is
either "controlled load" or "guaranteed service". The absence of
reservation can be indicated by aSKINg for the "best effort" service,
which is the default value of this parameter in a CreateConnection
command. For a ModifyConnection command, the default is simply to
retain the current value. When reservation has been asked on a
connection, the gateway will:
* start emitting RSVP "PATH" messages if the connection is in "send-
only", "send-receive", "conference", "network loop back" or
"network continuity test" mode (if a suitable remote connection
descriptor has been received,).
* start emitting RSVP "RESV" messages as soon as it receives "PATH"
messages if the connection is in "receive-only", "send-receive",
"conference", "network loop back" or "network continuity test"
mode.
The RSVP filters will be deduced from the characteristics of the
connection. The RSVP resource profiles will be deduced from the
connection's codecs, bandwidth and packetization period.
3. Media Gateway Control Protocol
The Media Gateway Control Protocol (MGCP) implements the media
gateway control interface as a set of transactions. The transactions
are composed of a command and a mandatory response. There are nine
commands:
* EndpointConfiguration
* CreateConnection
* ModifyConnection
* DeleteConnection
* NotificationRequest
* Notify
* AuditEndpoint
* AuditConnection
* RestartInProgress
The first five commands are sent by the Call Agent to a gateway. The
Notify command is sent by the gateway to the Call Agent. The gateway
may also send a DeleteConnection as defined in Section 2.3.8. The
Call Agent may send either of the Audit commands to the gateway, and
the gateway may send a RestartInProgress command to the Call Agent.
3.1 General Description
All commands are composed of a Command header, optionally followed by
a session description.
All responses are composed of a Response header, optionally followed
by session description information.
Headers and session descriptions are encoded as a set of text lines,
separated by a carriage return and line feed character (or,
optionally, a single line-feed character). The session descriptions
are preceded by an empty line.
MGCP uses a transaction identifier to correlate commands and
responses. The transaction identifier is encoded as a component of
the command header and repeated as a component of the response header
(see sections 3.2.1.2 and 3.3).
Note that an ABNF grammar for MGCP is provided in Appendix A.
Commands and responses SHALL be encoded in accordance with the
grammar, which, per RFC2234, is case-insensitive except for the SDP
part. Similarly, implementations SHALL be capable of decoding
commands and responses that follow the grammar. Additionally, it is
RECOMMENDED that implementations tolerate additional linear white
space.
Some productions allow for use of quoted strings, which can be
necessary to avoid syntax problems. Where the quoted string form is
used, the contents will be UTF-8 encoded [20], and the actual value
provided is the unquoted string (UTF-8 encoded). Where both a quoted
and unquoted string form is allowed, either form can be used provided
it does not otherwise violate the grammar.
In the following, we provide additional detail on the format of MGCP
commands and responses.
3.2 Command Header
The command header is composed of:
* A command line, identifying the requested action or verb, the
transaction identifier, the endpoint towards which the action is
requested, and the MGCP protocol version,
* A set of zero or more parameter lines, composed of a parameter
name followed by a parameter value.
Unless otherwise noted or dictated by other referenced standards
(e.g., SDP), each component in the command header is case
insensitive. This goes for verbs as well as parameters and values,
and hence all comparisons MUST treat upper and lower case as well as
combinations of these as being equal.
3.2.1 Command Line
The command line is composed of:
* The name of the requested verb,
* The identification of the transaction,
* The name of the endpoint(s) that are to execute the command (in
notifications or restarts, the name of the endpoint(s) that is
issuing the command),
* The protocol version.
These four items are encoded as strings of printable ASCII
characters, separated by white spaces, i.e., the ASCII space (0x20)
or tabulation (0x09) characters. It is RECOMMENDED to use exactly
one ASCII space separator. However, MGCP entities MUST be able to
parse messages with additional white space characters.
3.2.1.1 Coding of the Requested Verb
The verbs that can be requested are encoded as four letter upper or
lower case ASCII codes (comparisons SHALL be case insensitive) as
defined in the following table:
-----------------------------
Verb Code
----------------------------
EndpointConfiguration EPCF
CreateConnection CRCX
ModifyConnection MDCX
DeleteConnection DLCX
NotificationRequest RQNT
Notify NTFY
AuditEndpoint AUEP
AuditConnection AUCX
RestartInProgress RSIP
-----------------------------
The transaction identifier is encoded as a string of up to 9 decimal
digits. In the command line, it immediately follows the coding of
the verb.
New verbs may be defined in further versions of the protocol. It may
be necessary, for experimentation purposes, to use new verbs before
they are sanctioned in a published version of this protocol.
Experimental verbs MUST be identified by a four letter code starting
with the letter X, such as for example XPER.
3.2.1.2 Transaction Identifiers
MGCP uses a transaction identifier to correlate commands and
responses. A gateway supports two separate transaction identifier
name spaces:
* a transaction identifier name space for sending transactions, and
* a transaction identifier name space for receiving transactions.
At a minimum, transaction identifiers for commands sent to a given
gateway MUST be unique for the maximum lifetime of the transactions
within the collection of Call Agents that control that gateway.
Thus, regardless of the sending Call Agent, gateways can always
detect duplicate transactions by simply examining the transaction
identifier. The coordination of these transaction identifiers
between Call Agents is outside the scope of this specification
though.
Transaction identifiers for all commands sent from a given gateway
MUST be unique for the maximum lifetime of the transactions
regardless of which Call Agent the command is sent to. Thus, a Call
Agent can always detect a duplicate transaction from a gateway by the
combination of the domain-name of the endpoint and the transaction
identifier.
The transaction identifier is encoded as a string of up to nine
decimal digits. In the command lines, it immediately follows the
coding of the verb.
Transaction identifiers have values between 1 and 999,999,999 (both
included). Transaction identifiers SHOULD NOT use any leading
zeroes, although equality is based on numerical value, i.e., leading
zeroes are ignored. An MGCP entity MUST NOT reuse a transaction
identifier more quickly than three minutes after completion of the
previous command in which the identifier was used.
3.2.1.3 Coding of the Endpoint Identifiers and Entity Names
The endpoint identifiers and entity names are encoded as case
insensitive e-mail addresses, as defined in RFC821, although with
some syntactic restrictions on the local part of the name.
Furthermore, both the local endpoint name part and the domain name
part can each be up to 255 characters. In these addresses, the
domain name identifies the system where the endpoint is attached,
while the left side identifies a specific endpoint or entity on that
system.
Examples of such addresses are:
------------------------------------------------------------------
hrd4/56@gw23.example.net Circuit number 56 in
interface "hrd4" of the Gateway
23 of the "Example" network
Call-agent@ca.example.net Call Agent for the
"example" network
Busy-signal@ann12.example.net The "busy signal" virtual
endpoint in the announcement
server number 12.
------------------------------------------------------------------
The name of a notified entity is expressed with the same syntax, with
the possible addition of a port number as in:
Call-agent@ca.example.net:5234
In case the port number is omitted from the notified entity, the
default MGCP Call Agent port (2727) MUST be used.
3.2.1.4 Coding of the Protocol Version
The protocol version is coded as the keyword MGCP followed by a white
space and the version number, and optionally followed by a profile
name. The version number is composed of a major version, coded by a
decimal number, a dot, and a minor version number, coded as a decimal
number. The version described in this document is version 1.0.
The profile name, if present, is represented by white-space separated
strings of visible (printable) characters extending to the end of the
line. Profile names may be defined for user communities who want to
apply restrictions or other profiling to MGCP.
In the initial messages, the version will be coded as:
MGCP 1.0
An entity that receives a command with a protocol version it does not
support, MUST respond with an error (error code 528 - incompatible
protocol version, is RECOMMENDED). Note that this applies to
unsupported profiles as well.
3.2.2 Parameter Lines
Parameter lines are composed of a parameter name, which in most cases
is composed of one or two characters, followed by a colon, optional
white space(s) and the parameter value. The parameters that can be
present in commands are defined in the following table:
------------------------------------------------------------------
Parameter name Code Parameter value
----------------------------------------------------------------
BearerInformation B See description (3.2.2.1).
CallId C See description (3.2.2.2).
Capabilities A See description (3.2.2.3).
ConnectionId I See description (3.2.2.5).
ConnectionMode M See description (3.2.2.6).
ConnectionParameters P See description (3.2.2.7).
DetectEvents T See description (3.2.2.8).
DigitMap D A text encoding of a digit map.
EventStates ES See description (3.2.2.9).
LocalConnectionOptions L See description (3.2.2.10).
MaxMGCPDatagram MD See description (3.2.2.11).
NotifiedEntity N An identifier, in RFC821 format,
composed of an arbitrary string
and of the domain name of the
requesting entity, possibly com-
pleted by a port number, as in:
Call-agent@ca.example.net:5234
See also Section 3.2.1.3.
ObservedEvents O See description (3.2.2.12).
PackageList PL See description (3.2.2.13).
QuarantineHandling Q See description (3.2.2.14).
ReasonCode E A string with a 3 digit integer
optionally followed by a set of
arbitrary characters (3.2.2.15).
RequestedEvents R See description (3.2.2.16).
RequestedInfo F See description (3.2.2.17).
RequestIdentifier X See description (3.2.2.18).
ResponseAck K See description (3.2.2.19).
RestartDelay RD A number of seconds, encoded as
a decimal number.
RestartMethod RM See description (3.2.2.20).
SecondConnectionId I2 Connection Id.
SecondEndpointId Z2 Endpoint Id.
SignalRequests S See description (3.2.2.21).
SpecificEndPointId Z An identifier, in RFC821 format,
composed of an arbitrary string,
followed by an "@" followed by
the domain name of the gateway to
which this endpoint is attached.
See also Section 3.2.1.3.
----------------------------------------------------------------
RemoteConnection- RC Session Description.
Descriptor
LocalConnection- LC Session Description.
Descriptor
------------------------------------------------------------------
The parameters are not necessarily present in all commands. The
following table provides the association between parameters and
commands. The letter M stands for mandatory, O for optional and F
for forbidden. Unless otherwise specified, a parameter MUST NOT be
present more than once.
------------------------------------------------------------------
Parameter name EP CR MD DL RQ NT AU AU RS
CF CX CX CX NT FY EP CX IP
---------------------------------------------------------
BearerInformation O* O O O O F F F F
CallId F M M O F F F F F
Capabilities F F F F F F F F F
ConnectionId F F M O F F F M F
ConnectionMode F M O F F F F F F
Connection- F F F O* F F F F F
Parameters
DetectEvents F O O O O F F F F
DigitMap F O O O O F F F F
EventStates F F F F F F F F F
LocalConnection- F O O F F F F F F
Options
MaxMGCPDatagram F F F F F F F F F
NotifiedEntity F O O O O O F F F
ObservedEvents F F F F F M F F F
PackageList F F F F F F F F F
QuarantineHandling F O O O O F F F F
ReasonCode F F F O F F F F O
RequestedEvents F O O O O* F F F F
RequestIdentifier F O* O* O* M M F F F
RequestedInfo F F F F F F O M F
ResponseAck O O O O O O O O O
RestartDelay F F F F F F F F O
RestartMethod F F F F F F F F M
SecondConnectionId F F F F F F F F F
SecondEndpointId F O F F F F F F F
SignalRequests F O O O O* F F F F
SpecificEndpointId F F F F F F F F F
---------------------------------------------------------
RemoteConnection- F O O F F F F F F
Descriptor
LocalConnection- F F F F F F F F F
Descriptor
------------------------------------------------------------------
Notes (*):
* The BearerInformation parameter is only conditionally optional as
explained in Section 2.3.2.
* The RequestIdentifier parameter is optional in connection creation,
modification and deletion commands, however it becomes REQUIRED if
the command contains an encapsulated notification request.
* The RequestedEvents and SignalRequests parameters are optional in
the NotificationRequest. If these parameters are omitted the
corresponding lists will be considered empty.
* The ConnectionParameters parameter is only valid in a
DeleteConnection request sent by the gateway.
The set of parameters can be extended in two different ways:
* Package Extension Parameters (preferred)
* Vendor Extension Parameters
Package Extension Parameters are defined in packages which provides
the following benefits:
* a registration mechanism (IANA) for the package name.
* a separate name space for the parameters.
* a convenient grouping of the extensions.
* a simple way to determine support for them through auditing.
The package extension mechanism is the preferred extension method.
Vendor extension parameters can be used if implementers need to
experiment with new parameters, for example when developing a new
application of MGCP. Vendor extension parameters MUST be identified
by names that start with the string "X-" or "X+", such as for
example:
X-Flower: Daisy
Parameter names that start with "X+" are critical parameter
extensions. An MGCP entity that receives a critical parameter
extension that it cannot understand MUST refuse to execute the
command. It SHOULD respond with error code 511 (unrecognized
extension).
Parameter names that start with "X-" are non-critical parameter
extensions. An MGCP entity that receives a non-critical parameter
extension that it cannot understand MUST simply ignore that
parameter.
Note that vendor extension parameters use an unmanaged name space,
which implies a potential for name clashing. Vendors are
consequently encouraged to include some vendor specific string, e.g.,
vendor name, in their vendor extensions.
3.2.2.1 BearerInformation
The values of the bearer information are encoded as a comma separated
list of attributes, which are represented by an attribute name, and
possibly followed by a colon and an attribute value.
The only attribute that is defined is the "encoding" (code "e")
attribute, which MUST have one of the values "A" (A-law) or "mu"
(mu-law).
An example of bearer information encoding is:
B: e:mu
The set of bearer information attributes may be extended through
packages.
3.2.2.2 CallId
The Call Identifier is encoded as a hexadecimal string, at most 32
characters in length. Call Identifiers are compared as strings
rather than numerical values.
3.2.2.3 Capabilities
Capabilities inform the Call Agent about endpoints' capabilities when
audited. The encoding of capabilities is based on the Local
Connection Options encoding for the parameters that are common to
both, although a different parameter line code is used ("A"). In
addition, capabilities can also contain a list of supported packages,
and a list of supported modes.
The parameters used are:
A list of supported codecs.
The following parameters will apply to all codecs specified in
this list. If there is a need to specify that some parameters,
such as e.g., silence suppression, are only compatible with some
codecs, then the gateway will return several Capability
parameters; one for each set of codecs.
Packetization Period:
A range may be specified.
Bandwidth:
A range corresponding to the range for packetization periods may
be specified (assuming no silence suppression). If absent, the
values will be deduced from the codec type.
Echo Cancellation:
"on" if echo cancellation is supported, "off" otherwise. The
default is support.
Silence Suppression:
"on" if silence suppression is supported for this codec, "off"
otherwise. The default is support.
Gain Control:
"0" if gain control is not supported, all other values indicate
support for gain control. The default is support.
Type of Service:
The value "0" indicates no support for type of service, all other
values indicate support for type of service. The default is
support.
Resource Reservation Service:
The parameter indicates the reservation services that are
supported, in addition to best effort. The value "g" is encoded
when the gateway supports both the guaranteed and the controlled
load service, "cl" when only the controlled load service is
supported. The default is "best effort".
Encryption Key:
Encoding any value indicates support for encryption. Default is
no support which is implied by omitting the parameter.
Type of network:
The keyword "nt", followed by a colon and a semicolon separated
list of supported network types. This parameter is optional.
Packages:
The packages supported by the endpoint encoded as the keyword "v",
followed by a colon and a character string. If a list of values
is specified, these values will be separated by a semicolon. The
first value specified will be the default package for the
endpoint.
Modes:
The modes supported by this endpoint encoded as the keyword "m",
followed by a colon and a semicolon-separated list of supported
connection modes for this endpoint.
Lack of support for a capability can also be indicated by excluding
the parameter from the capability set.
An example capability is:
A: a:PCMU;G728, p:10-100, e:on, s:off, t:1, v:L,
m:sendonly;recvonly;sendrecv;inactive
The carriage return above is included for formatting reasons only and
is not permissible in a real implementation.
If multiple capabilities are to be returned, each will be returned as
a separate capability line.
Since Local Connection Options can be extended, the list of
capability parameters can also be extended. Individual extensions
may define how they are reported as capabilities. If no such
definition is provided, the following defaults apply:
* Package Extension attributes: The individual attributes are not
reported. Instead, the name of the package is simply reported in
the list of supported packages.
* Vendor Extension attributes: The name of the attribute is reported
without any value.
* Other Extension attributes: The name of the attribute is reported
without any value.
3.2.2.4 Coding of Event Names
Event names are composed of an optional package name, separated by a
slash (/) from the name of the actual event (see Section 2.1.7). The
wildcard character star ("*") can be use to refer to all packages.
The event name can optionally be followed by an at sign (@) and the
identifier of a connection (possibly using a wildcard) on which the
event should be observed. Event names are used in the
RequestedEvents, SignalRequests, ObservedEvents, DetectEvents, and
EventStates parameters.
Events and signals may be qualified by parameters defined for the
event/signal. Such parameters may be enclosed in double-quotes (in
fact, some parameters MUST be enclosed in double-quotes due to
syntactic restrictions) in which case they are UTF-8 encoded [20].
The parameter name "!" (exclamation point) is reserved for future use
for both events and signals.
Each signal has one of the following signal-types associated with it:
On/Off (OO), Time-out (TO), or Brief (BR). (These signal types are
specified in the package definitions, and are not present in the
messages.) On/Off signals can be parameterized with a "+" to turn the
signal on, or a "-" to turn the signal off. If an on/off signal is
not parameterized, the signal is turned on. Both of the following
will turn the vmwi signal (from the line package "L") on:
L/vmwi(+)
L/vmwi
In addition to "!", "+" and "-", the signal parameter "to" is
reserved as well. It can be used with Time-Out signals to override
the default time-out value for the current request. A decimal value
in milliseconds will be supplied. The individual signal and/or
package definition SHOULD indicate if this parameter is supported for
one or more TO signals in the package. If not indicated, TO signals
in package version zero are assumed to not support it, whereas TO
signals in package versions one or higher are assumed to support it.
By default, a supplied time-out value MAY be rounded to the nearest
non-zero value divisible by 1000, i.e., whole second. The individual
signal and/or package definition may define other rounding rules. All
new package and TO signal definitions are strongly encouraged to
support the "to" signal parameter.
The following example illustrates how the "to" parameter can be used
to apply a signal for 6 seconds:
L/rg(to=6000)
L/rg(to(6000))
The following are examples of event names:
-----------------------------------------------------------
L/hu on-hook transition, in the line package
F/0 digit 0 in the MF package
hf Hook-flash, assuming that the line package
is the default package for the endpoint.
G/rt@0A3F58 Ring back signal on connection "0A3F58"
-----------------------------------------------------------
In addition, the range and wildcard notation of events can be used,
instead of individual names, in the RequestedEvents and DetectEvents
parameters. The event code "all" is reserved and refers to all
events or signals in a package. The star sign ("*") can be used to
denote "all connections", and the dollar sign ("$") can be used to
denote the "current" connection (see Section 2.1.7 for details).
The following are examples of such notations:
---------------------------------------------------------
M/[0-9] Digits 0 to 9 in the MF package.
hf Hook-flash, assuming that the line package
is a default package for the endpoint.
[0-9*#A-D] All digits and letters in the DTMF
packages (default for endpoint).
T/all All events in the trunk package.
R/qa@* The quality alert event on all
connections.
G/rt@$ Ringback on current connection.
---------------------------------------------------------
3.2.2.5 ConnectionId
The Connection Identifier is encoded as a hexadecimal string, at most
32 characters in length. Connection Identifiers are compared as
strings rather than numerical values.
3.2.2.6 ConnectionMode
The connection mode describes the mode of operation of the
connection. The possible values are:
--------------------------------------------------------
Mode Meaning
-------------------------------------------------------
M: sendonly The gateway should only send packets
M: recvonly The gateway should only receive packets
M: sendrecv The gateway should send
and receive packets
M: confrnce The gateway should place
the connection in conference mode
M: inactive The gateway should neither
send nor receive packets
M: loopback The gateway should place
the circuit in loopback mode.
M: conttest The gateway should place
the circuit in test mode.
M: netwloop The gateway should place
the connection in network loopback mode.
M: netwtest The gateway should place the connection
in network continuity test mode.
--------------------------------------------------------
Note that irrespective of the connection mode, signals applied to the
connection will still result in packets being sent (see Section
2.3.1).
The set of connection modes can be extended through packages.
3.2.2.7 ConnectionParameters
Connection parameters are encoded as a string of type and value
pairs, where the type is either a two-letter identifier of the
parameter or an extension type, and the value a decimal integer.
Types are separated from value by an '=' sign. Parameters are
separated from each other by a comma. Connection parameter values
can contain up to nine digits. If the maximum value is reached, the
counter is no longer updated, i.e., it doesn't wrap or overflow.
The connection parameter types are specified in the following table:
-----------------------------------------------------------------
Connection parameter Code Connection parameter
name value
---------------------------------------------------------------
Packets sent PS The number of packets that
were sent on the connection.
Octets sent OS The number of octets that
were sent on the connection.
Packets received PR The number of packets that
were received on the connection.
Octets received OR The number of octets that
were received on the connection.
Packets lost PL The number of packets that
were lost on the connection
as deduced from gaps in the
RTP sequence number.
Jitter JI The average inter-packet arrival
jitter, in milliseconds,
expressed as an integer number.
Latency LA Average latency, in milliseconds,
expressed as an integer number.
-----------------------------------------------------------------
The set of connection parameters can be extended in two different
ways:
* Package Extension Parameters (preferred)
* Vendor Extension Parameters
Package Extension Connection Parameters are defined in packages which
provides the following benefits:
* A registration mechanism (IANA) for the package name.
* A separate name space for the parameters.
* A convenient grouping of the extensions.
* A simple way to determine support for them through auditing.
The package extension mechanism is the preferred extension method.
Vendor extension parameters names are composed of the string "X-"
followed by a two or more letters extension parameter name.
Call agents that receive unrecognized package or vendor connection
parameter extensions SHALL silently ignore these parameters.
An example of connection parameter encoding is:
P: PS=1245, OS=62345, PR=0, OR=0, PL=0, JI=0, LA=48
3.2.2.8 DetectEvents
The DetectEvents parameter is encoded as a comma separated list of
events (see Section 3.2.2.4), such as for example:
T: L/hu,L/hd,L/hf,D/[0-9#*]
It should be noted, that no actions can be associated with the
events, however event parameters may be provided.
3.2.2.9 EventStates
The EventStates parameter is encoded as a comma separated list of
events (see Section 3.2.2.4), such as for example:
ES: L/hu
It should be noted, that no actions can be associated with the
events, however event parameters may be provided.
3.2.2.10 LocalConnectionOptions
The local connection options describe the operational parameters that
the Call Agent provides to the gateway in connection handling
commands. These include:
* The allowed codec(s), encoded as the keyword "a", followed by a
colon and a character string. If the Call Agent specifies a list
of values, these values will be separated by a semicolon. For RTP,
audio codecs SHALL be specified by using encoding names defined in
the RTP AV Profile [4] or its replacement, or by encoding names
registered with the IANA. Non-audio media registered as a MIME
type MUST use the "<MIME type>/<MIME subtype>" form, as in
"image/t38".
* The packetization period in milliseconds, encoded as the keyword
"p", followed by a colon and a decimal number. If the Call Agent
specifies a range of values, the range will be specified as two
decimal numbers separated by a hyphen (as specified for the "ptime"
parameter for SDP).
* The bandwidth in kilobits per second (1000 bits per second),
encoded as the keyword "b", followed by a colon and a decimal
number. If the Call Agent specifies a range of values, the range
will be specified as two decimal numbers separated by a hyphen.
* The type of service parameter, encoded as the keyword "t", followed
by a colon and the value encoded as two hexadecimal digits. When
the connection is transmitted over an IP network, the parameters
encode the 8-bit type of service value parameter of the IP header
(a.k.a. DiffServ field). The left-most "bit" in the parameter
corresponds to the least significant bit in the IP header.
* The echo cancellation parameter, encoded as the keyword "e",
followed by a colon and the value "on" or "off".
* The gain control parameter, encoded as the keyword "gc", followed
by a colon and a value which can be either the keyword "auto" or a
decimal number (positive or negative) representing the number of
decibels of gain.
* The silence suppression parameter, encoded as the keyword "s",
followed by a colon and the value "on" or "off".
* The resource reservation parameter, encoded as the keyword "r",
followed by a colon and the value "g" (guaranteed service), "cl"
(controlled load) or "be" (best effort).
* The encryption key, encoded as the keyword "k" followed by a colon
and a key specification, as defined for the parameter "K" in SDP
(RFC2327).
* The type of network, encoded as the keyword "nt" followed by a
colon and the type of network encoded as the keyword "IN"
(internet), "ATM", "LOCAL" (for a local connection), or possibly
another type of network registered with the IANA as per SDP (RFC
2327).
* The resource reservation parameter, encoded as the keyword "r",
followed by a colon and the value "g" (guaranteed service), "cl"
(controlled load) or "be" (best effort).
The encoding of the first three attributes, when they are present,
will be compatible with the SDP and RTP profiles. Note that each of
the attributes is optional. When several attributes are present,
they are separated by a comma.
Examples of local connection options are:
L: p:10, a:PCMU
L: p:10, a:G726-32
L: p:10-20, b:64
L: b:32-64, e:off
The set of Local Connection Options attributes can be extended in
three different ways:
* Package Extension attributes (preferred)
* Vendor Extension attributes
* Other Extension attributes
Package Extension Local Connection Options attributes are defined in
packages which provides the following benefits:
* A registration mechanism (IANA) for the package name.
* A separate name space for the attributes.
* A convenient grouping of the extensions.
* A simple way to determine support for them through auditing.
The package extension mechanism is the preferred extension method.
Vendor extension attributes are composed of an attribute name, and
possibly followed by a colon and an attribute value. The attribute
name MUST start with the two characters "x+", for a mandatory
extension, or "x-", for a non-mandatory extension. If a gateway
receives a mandatory extension attribute that it does not recognize,
it MUST reject the command (error code 525 - unknown extension in
LocalConnectionOptions, is RECOMMENDED).
Note that vendor extension attributes use an unmanaged name space,
which implies a potential for name clashing. Vendors are
consequently encouraged to include some vendor specific string, e.g.,
vendor name, in their vendor extensions.
Finally, for backwards compatibility with some existing
implementations, MGCP allows for other extension attributes as well
(see grammar in Appendix A). Note however, that these attribute
extensions do not provide the package extension attribute benefits.
Use of this mechanism for new extensions is discouraged.
3.2.2.11 MaxMGCPDatagram
The MaxMGCPDatagram can only be used for auditing, i.e., it is a
valid RequestedInfo code and can be provided as a response parameter.
In responses, the MaxMGCPDatagram value is encoded as a string of up
to nine decimal digits -- leading zeroes are not permitted. The
following example illustrates the use of this parameter:
MD: 8100
3.2.2.12 ObservedEvents
The observed events parameter provides the list of events that have
been observed. The event codes are the same as those used in the
NotificationRequest. Events that have been accumulated according to
the digit map may be grouped in a single string, however such
practice is discouraged; they SHOULD be reported as lists of isolated
events if other events were detected during the digit accumulation.
Examples of observed events are:
O: L/hu
O: D/8295555T
O: D/8,D/2,D/9,D/5,D/5,L/hf,D/5,D/5,D/T
O: L/hf, L/hf, L/hu
3.2.2.13 PackageList
The Package List can only be used for auditing, i.e., it is a valid
RequestedInfo code and can be provided as a response parameter.
The response parameter will consist of a comma separated list of
packages supported. The first package returned in the list is the
default package. Each package in the list consists of the package
name followed by a colon, and the highest version number of the
package supported.
An example of a package list is:
PL: L:1,G:1,D:0,FOO:2,T:1
Note that for backwards compatibility, support for this parameter is
OPTIONAL.
3.2.2.14 QuarantineHandling
The quarantine handling parameter contains a list of comma separated
keywords:
* The keyword "process" or "discard" to indicate the treatment of
quarantined and observed events. If neither "process" or "discard"
is present, "process" is assumed.
* The keyword "step" or "loop" to indicate whether at most one
notification per NotificationRequest is allowed, or whether
multiple notifications per NotificationRequest are allowed. If
neither "step" nor "loop" is present, "step" is assumed.
The following values are valid examples:
Q: loop
Q: process
Q: loop,discard
3.2.2.15 ReasonCode
Reason codes are three-digit numeric values. The reason code is
optionally followed by a white space and commentary, e.g.:
E: 900 Endpoint malfunctioning
A list of reason codes can be found in Section 2.5.
The set of reason codes can be extended through packages.
3.2.2.16 RequestedEvents
The RequestedEvents parameter provides the list of events that are
requested. The event codes are described in Section 3.2.2.4.
Each event can be qualified by a requested action, or by a list of
actions. The actions, when specified, are encoded as a list of
keywords, enclosed in parenthesis and separated by commas. The codes
for the various actions are:
-------------------------------------
Action Code
------------------------------------
Notify immediately N
Accumulate A
Treat according to digit map D
Swap S
Ignore I
Keep Signal(s) active K
Embedded Notification Request E
-------------------------------------
When no action is specified, the default action is to notify the
event. This means that, for example, ft and ft(N) are equivalent.
Events that are not listed are ignored (unless they are persistent).
The digit-map action SHOULD only be specified for the digits, letters
and interdigit timers in packages that define the encoding of digits,
letters, and timers (including extension digit map letters).
The requested events list is encoded on a single line, with
event/action groups separated by commas. Examples of RequestedEvents
encodings are:
R: L/hu(N), L/hf(S,N)
R: L/hu(N), D/[0-9#T](D)
In the case of the "Embedded Notification Request" action, the
embedded notification request parameters are encoded as a list of up
to three parameter groups separated by commas. Each group starts by
a one letter identifier, followed by a list of parameters enclosed
between parentheses. The first optional parameter group, identified
by the letter "R", is the value of the embedded RequestedEvents
parameter. The second optional group, identified by the letter "S",
is the embedded value of the SignalRequests parameter. The third
optional group, identified by the letter "D", is the embedded value
of the DigitMap. (Note that some existing implementations and
profiles may encode these three components in a different order.
Implementers are encouraged to accept such encodings, but they SHOULD
NOT generate them.)
If the RequestedEvents parameter is not present, the parameter will
be set to a null value. If the SignalRequests parameter is not
present, the parameter will be set to a null value. If the DigitMap
is absent, the current value MUST be used. The following are valid
examples of embedded requests:
R: L/hd(E(R(D/[0-9#T](D),L/hu(N)),S(L/dl),D([0-9].[#T])))
R: L/hd(E(R(D/[0-9#T](D),L/hu(N)),S(L/dl)))
Some events can be qualified by additional event parameters. Such
event parameters will be separated by commas and enclosed within
parentheses. Event parameters may be enclosed in double-quotes (in
fact, some event parameters MUST be enclosed in double-quotes due to
syntactic restrictions), in which case the quoted string itself is
UTF-8 encoded. Please refer to Section 3.2.2.4 for additional detail
on event parameters.
The following example shows the Foobar event with an event parameter
"epar":
R: X/foobar(N)(epar=2)
Notice that the Action was included even though it is the default
Notify action - this is required by the grammar.
3.2.2.17 RequestedInfo
The RequestedInfo parameter contains a comma separated list of
parameter codes, as defined in Section 3.2.2. For example, if one
wants to audit the value of the NotifiedEntity, RequestIdentifier,
RequestedEvents, SignalRequests, DigitMap, QuarantineHandling and
DetectEvents parameters, the value of the RequestedInfo parameter
will be:
F: N,X,R,S,D,Q,T
Note that extension parameters in general can be audited as well.
The individual extension will define the auditing operation.
The capabilities request, in the AuditEndPoint command, is encoded by
the parameter code "A", as in:
F: A
3.2.2.18 RequestIdentifier
The request identifier correlates a Notify command with the
NotificationRequest that triggered it. A RequestIdentifier is a
hexadecimal string, at most 32 characters in length.
RequestIdentifiers are compared as strings rather than numerical
value. The string "0" is reserved for reporting of persistent events
in the case where a NotificationRequest has not yet been received
after restart.
3.2.2.19 ResponseAck
The response acknowledgement parameter is used to manage the "at-
most-once" facility described in Section 3.5. It contains a comma
separated list of "confirmed transaction-id ranges".
Each "confirmed transaction-id range" is composed of either one
decimal number, when the range includes exactly one transaction, or
two decimal numbers separated by a single hyphen, describing the
lower and higher transaction identifiers included in the range.
An example of a response acknowledgement is:
K: 6234-6255, 6257, 19030-19044
3.2.2.20 RestartMethod
The RestartMethod parameter is encoded as one of the keywords
"graceful", "forced", "restart", "disconnected" or "cancel-graceful"
as for example:
RM: restart
The set of restart methods can be extended through packages.
3.2.2.21 SignalRequests
The SignalRequests parameter provides the name of the signal(s) that
have been requested. Each signal is identified by a name, as
described in Section 3.2.2.4.
Some signals, such as for example announcement or ADSI display, can
be qualified by additional parameters, e.g.:
* the name and parameters of the announcement,
* the string that should be displayed.
Such parameters will be separated by commas and enclosed within
parenthesis, as in:
S: L/adsi("123456 Francois Gerard")
S: A/ann(http://ann.example.net/no-such-number.au, 1234567)
When a quoted-string is provided, the string itself is UTF-8 encoded
[20].
When several signals are requested, their codes are separated by a
comma, as in:
S: L/adsi("123456 Your friend"), L/rg
Please refer to Section 3.2.2.4 for additional detail on signal
parameters.
3.3 Format of response headers
The response header is composed of a response line, optionally
followed by headers that encode the response parameters.
An example of a response header could be:
200 1203 OK
The response line starts with the response code, which is a three
digit numeric value. The code is followed by a white space, and the
transaction identifier. Response codes defined in packages (8xx) are
followed by white space, a slash ("/") and the package name. All
response codes may furthermore be followed by optional commentary
preceded by a white space.
The following table describes the parameters whose presence is
mandatory or optional in a response header, as a function of the
command that triggered the response. The letter M stands for
mandatory, O for optional and F for forbidden. Unless otherwise
specified, a parameter MUST NOT be present more than once. Note that
the table only reflects the default for responses that have not
defined any other behavior. If a response is received with a
parameter that is either not understood or marked as forbidden, the
offending parameter(s) MUST simply be ignored.
------------------------------------------------------------------
Parameter name EP CR MD DL RQ NT AU AU RS
CF CX CX CX NT FY EP CX IP
---------------------------------------------------------
BearerInformation F F F F F F O F F
CallId F F F F F F F O F
Capabilities F F F F F F O* F F
ConnectionId F O* F F F F O* F F
ConnectionMode F F F F F F F O F
Connection- F F F O* F F F O F
Parameters
DetectEvents F F F F F F O F F
DigitMap F F F F F F O F F
EventStates F F F F F F O F F
LocalConnection- F F F F F F F O F
Options
MaxMGCPDatagram F F F F F F O F F
NotifiedEntity F F F F F F O O O
ObservedEvents F F F F F F O F F
QuarantineHandling F F F F F F O F F
PackageList O* O* O* O* O* O* O O* O*
ReasonCode F F F F F F O F F
RequestIdentifier F F F F F F O F F
ResponseAck O* O* O* O* O* O* O* O* O*
RestartDelay F F F F F F O F F
RestartMethod F F F F F F O F F
RequestedEvents F F F F F F O F F
RequestedInfo F F F F F F F F F
SecondConnectionId F O F F F F F F F
SecondEndpointId F O F F F F F F F
SignalRequests F F F F F F O F F
SpecificEndpointId F O F F F F O* F F
---------------------------------------------------------
LocalConnection- F O* O F F F F O* F
Descriptor
RemoteConnection- F F F F F F F O* F
Descriptor
------------------------------------------------------------------
Notes (*):
* The PackageList parameter is only allowed with return code 518
(unsupported package), except for AuditEndpoint, where it may also
be returned if audited.
* The ResponseAck parameter MUST NOT be used with any other responses
than a final response issued after a provisional response for the
transaction in question. In that case, the presence of the
ResponseAck parameter SHOULD trigger a Response Acknowledgement -
any ResponseAck values provided will be ignored.
* In the case of a CreateConnection message, the response line is
followed by a Connection-Id parameter and a
LocalConnectionDescriptor. It may also be followed a Specific-
Endpoint-Id parameter, if the creation request was sent to a
wildcarded Endpoint-Id. The connection-Id and
LocalConnectionDescriptor parameter are marked as optional in the
Table. In fact, they are mandatory with all positive responses,
when a connection was created, and forbidden when the response is
negative, and no connection was created.
* A LocalConnectionDescriptor MUST be transmitted with a positive
response (code 200) to a CreateConnection. It MUST also be
transmitted in response to a ModifyConnection command, if the
modification resulted in a modification of the session parameters.
The LocalConnectionDescriptor is encoded as a "session
description", as defined in section 3.4. It is separated from the
response header by an empty line.
* Connection-Parameters are only valid in a response to a non-
wildcarded DeleteConnection command sent by the Call Agent.
* Multiple ConnectionId, SpecificEndpointId, and Capabilities
parameters may be present in the response to an AuditEndpoint
command.
* When several session descriptors are encoded in the same response,
they are encoded one after each other, separated by an empty line.
This is the case for example when the response to an audit
connection request carries both a local session description and a
remote session description, as in:
200 1203 OK
C: A3C47F21456789F0
N: [128.96.41.12]
L: p:10, a:PCMU;G726-32
M: sendrecv
P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27,LA=48
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=-
c=IN IP4 128.96.41.1
t=0 0
m=audio 1296 RTP/AVP 0
v=0
o=- 33343 346463 IN IP4 128.96.63.25
s=-
c=IN IP4 128.96.63.25
t=0 0
m=audio 1296 RTP/AVP 0 96
a=rtpmap:96 G726-32/8000
In this example, according to the SDP syntax, each description
starts with a "version" line, (v=...). The local description is
always transmitted before the remote description. If a connection
descriptor is requested, but it does not exist for the connection
audited, that connection descriptor will appear with the SDP
protocol version field only.
The response parameters are described for each of the commands in the
following.
3.3.1 CreateConnection Response
In the case of a CreateConnection message, the response line is
followed by a Connection-Id parameter with a successful response
(code 200). A LocalConnectionDescriptor is furthermore transmitted
with a positive response. The LocalConnectionDescriptor is encoded
as a "session description", as defined by SDP (RFC2327). It is
separated from the response header by an empty line, e.g.:
200 1204 OK
I: FDE234C8
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=-
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 96
a=rtpmap:96 G726-32/8000
When a provisional response has been issued previously, the final
response SHOULD furthermore contain the Response Acknowledgement
parameter (final responses issued by entities adhering to this
specification will include the parameter, but older RFC2705
implementations MAY not):
200 1204 OK
K:
I: FDE234C8
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=-
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 96
a=rtpmap:96 G726-32/8000
The final response SHOULD then be acknowledged by a Response
Acknowledgement:
000 1204
3.3.2 ModifyConnection Response
In the case of a successful ModifyConnection message, the response
line is followed by a LocalConnectionDescriptor, if the modification
resulted in a modification of the session parameters (e.g., changing
only the mode of a connection does not alter the session parameters).
The LocalConnectionDescriptor is encoded as a "session description",
as defined by SDP. It is separated from the response header by an
empty line.
200 1207 OK
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=-
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 0
When a provisional response has been issued previously, the final
response SHOULD furthermore contain the Response Acknowledgement
parameter as in:
200 1207 OK
K:
The final response SHOULD then be acknowledged by a Response
Acknowledgement:
000 1207 OK
3.3.3 DeleteConnection Response
Depending on the variant of the DeleteConnection message, the
response line may be followed by a Connection Parameters parameter
line, as defined in Section 3.2.2.7.
250 1210 OK
P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27, LA=48
3.3.4 NotificationRequest Response
A successful NotificationRequest response does not include any
additional response parameters.
3.3.5 Notify Response
A successful Notify response does not include any additional response
parameters.
3.3.6 AuditEndpoint Response
In the case of a successful AuditEndPoint the response line may be
followed by information for each of the parameters requested - each
parameter will appear on a separate line. Parameters for which no
value currently exists, e.g., digit map, will still be provided but
with an empty value. Each local endpoint name "expanded" by a
wildcard character will appear on a separate line using the
"SpecificEndPointId" parameter code, e.g.:
200 1200 OK
Z: aaln/1@rgw.whatever.net
Z: aaln/2@rgw.whatever.net
When connection identifiers are audited and multiple connections
exist on the endpoint, a comma-separated list of connection
identifiers SHOULD be returned as in:
200 1200 OK
I: FDE234C8, DFE233D1
Alternatively, multiple connection id parameter lines may be returned
- the two forms should not be mixed although doing so does not
constitute an error.
When capabilities are audited, the response may include multiple
capabilities parameter lines as in:
200 1200 OK
A: a:PCMU;G728, p:10-100, e:on, s:off, t:1, v:L,
m:sendonly;recvonly;sendrecv;inactive
A: a:G729, p:30-90, e:on, s:on, t:1, v:L,
m:sendonly;recvonly;sendrecv;inactive;confrnce
Note: The carriage return for Capabilities shown above is present
for formatting reasons only. It is not permissible in a real command
encoding.
3.3.7 AuditConnection Response
In the case of a successful AuditConnection, the response may be
followed by information for each of the parameters requested.
Parameters for which no value currently exists will still be
provided. Connection descriptors will always appear last and each
will be preceded by an empty line, as for example:
200 1203 OK
C: A3C47F21456789F0
N: [128.96.41.12]
L: p:10, a:PCMU;G728
M: sendrecv
P: PS=622, OS=31172, PR=390, OR=22561, PL=5, JI=29, LA=50
v=0
o=- 4723891 7428910 IN IP4 128.96.63.25
s=-
c=IN IP4 128.96.63.25
t=0 0
m=audio 1296 RTP/AVP 96
a=rtpmap:96 G726-32/8000
If both a local and a remote connection descriptor are provided, the
local connection descriptor will be the first of the two. If a
connection descriptor is requested, but it does not exist for the
connection audited, that connection descriptor will appear with the
SDP protocol version field only ("v=0"), as for example:
200 1203 OK
v=0
3.3.8 RestartInProgress Response
A successful RestartInProgress response may include a NotifiedEntity
parameter, but otherwise does not include any additional response
parameters.
Also, a 521 response to a RestartInProgress MUST include a
NotifiedEntity parameter with the name of another Call Agent to
contact when the first Call Agent redirects the endpoint to another
Call Agent as in:
521 1204 Redirect
N: CA-1@whatever.net
3.4 Encoding of the Session Description (SDP)
The session description (SDP) is encoded in conformance with the
session description protocol, SDP. MGCP implementations are REQUIRED
to be fully capable of parsing any conformant SDP message, and MUST
send session descriptions that strictly conform to the SDP standard.
The general description and explanation of SDP parameters can be
found in RFC2327 (or its successor). In particular, it should be
noted that the
* Origin ("o="),
* Session Name ("s="), and
* Time active ("t=")
are all mandatory in RFC2327. While they are of little use to MGCP,
they MUST be provided in conformance with RFC2327 nevertheless. The
following suggests values to be used for each of the fields, however
the reader is encouraged to consult RFC2327 (or its successor) for
details:
Origin
o = <username> <session id> <version> <network type> <address type>
<address>
* The username SHOULD be set to hyphen ("-").
* The session id is RECOMMENDED to be an NTP timestamp as suggested
in RFC2327.
* The version is a version number that MUST increment with each
change to the SDP. A counter initialized to zero or an NTP
timestamp as suggested in RFC2327 is RECOMMENDED.
* The network type defines the type of network. For RTP sessions the
network type SHOULD be "IN".
* The address type defines the type of address. For RTP sessions the
address type SHOULD be "IP4" (or "IP6").
* The address SHOULD be the same address as provided in the
connection information ("c=") field.
Session Name
s = <session name>
The session name should be hyphen ("-").
Time active
t = <start time> <stop time>
* The start time may be set to zero.
* The stop time should be set to zero.
Each of the three fields can be ignored upon reception.
To further accommodate the extensibility principles of MGCP,
implementations are ENCOURAGED to support the PINT "a=require"
attribute - please refer to RFC2848 for further details.
The usage of SDP actually depends on the type of session that is
being established. Below we describe usage of SDP for an audio
service using the RTP/AVP profile [4], or the LOCAL interconnect
defined in this document. In case of any conflicts between what is
described below and SDP (RFC2327 or its successor), the SDP
specification takes precedence.
3.4.1 Usage of SDP for an Audio Service
In a telephony gateway, we only have to describe sessions that use
exactly one media, audio. The usage of SDP for this is
straightforward and described in detail in RFC2327.
The following is an example of an RFC2327 conformant session
description for an audio connection:
v=0
o=- A7453949499 0 IN IP4 128.96.41.1
s=-
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 0 96
a=rtpmap:96 G726-32/8000
3.4.2 Usage of SDP for LOCAL Connections
When MGCP is used to set up internal connections within a single
gateway, the SDP format is used to encode the parameters of that
connection. The connection and media parameters will be used as
follows:
* The connection parameter (c=) will specify that the connection is
local, using the keyword "LOCAL" as network type, the keyword "EPN"
(endpoint name) as address type, and the local name of the endpoint
as the connection-address.
* The "m=audio" parameter will specify a port number, which will
always be set to 0, the type of protocol, always set to the keyword
LOCAL, and the type of encoding, using the same conventions used
for the RTP AVP profile (RTP payload numbers). The type of
encoding should normally be set to 0 (PCMU).
A session-level attribute identifying the connection MAY furthermore
be present. This enables endpoints to support multiple LOCAL
connections. Use of this attribute is OPTIONAL and indeed
unnecessary for endpoints that only support a single LOCAL
connection. The attribute is defined as follows:
a=MGCPlocalcx:<ConnectionID>
The MGCP Local Connection attribute is a session level only case-
insensitive attribute that identifies the MGCP LOCAL connection,
on the endpoint identified in the connection information, to which
the SDP applies. The ConnectionId is a hexadecimal string
containing at most 32 characters. The ConnectionId itself is
case-insensitive. The MGCP Local Connection attribute is not
subject to the charset attribute.
An example of a LOCAL session description could be:
v=0
o=- A7453949499 0 LOCAL EPN X35V3+A4/13
s=-
c=LOCAL EPN X35V3+A4/13
t=0 0
a=MGCPlocalcx:FDE234C8
m=audio 0 LOCAL 0
Note that the MGCP Local Connection attribute is specified at the
session level and that it could have been omitted in case only a
single LOCAL connection per endpoint is supported.
3.5 Transmission over UDP
MGCP messages are transmitted over UDP. Commands are sent to one of
the IP addresses defined in the DNS for the specified endpoint. The
responses are sent back to the source address (i.e., IP address and
UDP port number) of the commands - the response may or may not arrive
from the same address as the command was sent to.
When no port is specified for the endpoint, the commands MUST by
default be sent:
* by the Call Agents, to the default MGCP port for gateways, 2427.
* by the Gateways, to the default MGCP port for Call Agents, 2727.
3.5.1 Providing the At-Most-Once Functionality
MGCP messages, being carried over UDP, may be subject to losses. In
the absence of a timely response, commands are retransmitted. Most
MGCP commands are not idempotent. The state of the gateway would
become unpredictable if, for example, CreateConnection commands were
executed several times. The transmission procedures MUST thus
provide an "at-most-once" functionality.
MGCP entities are expected to keep in memory a list of the responses
that they sent to recent transactions, and a list of the transactions
that are currently being executed. The numerical value of
transaction identifiers of incoming commands are compared to the
transaction identifiers of the recent responses. If a match is
found, the MGCP entity does not execute the transaction again, but
simply resends the response. The remaining commands will be compared
to the list of current transactions, i.e., transactions received
previously which have not yet finished executing. If a match is
found, the MGCP entity does not execute the transaction again, but a
provisional response (Section 3.5.5) SHOULD be issued to acknowledge
receipt of the command.
The procedure uses a long timer value, noted T-HIST in the following.
The timer MUST be set larger than the maximum duration of a
transaction, which MUST take into account the maximum number of
repetitions, the maximum value of the repetition timer and the
maximum propagation delay of a packet in the network. A suggested
value is 30 seconds.
The copy of the responses MAY be destroyed either T-HIST seconds
after the response is issued, or when the gateway (or the Call Agent)
receives a confirmation that the response has been received, through
the "Response Acknowledgement". For transactions that are
acknowledged through this attribute, the gateway SHALL keep a copy of
the transaction-id (as opposed to the entire transaction response)
for T-HIST seconds after the response is issued, in order to detect
and ignore duplicate copies of the transaction request that could be
produced by the network.
3.5.2 Transaction Identifiers and Three Ways Handshake
Transaction identifiers are integer numbers in the range from 1 to
999,999,999 (both included). Call-agents may decide to use a
specific number space for each of the gateways that they manage, or
to use the same number space for all gateways that belong to some
arbitrary group. Call agents may decide to share the load of
managing a large gateway between several independent processes.
These processes MUST then share the transaction number space. There
are multiple possible implementations of this sharing, such as having
a centralized allocation of transaction identifiers, or pre-
allocating non-overlapping ranges of identifiers to different
processes. The implementations MUST guarantee that unique
transaction identifiers are allocated to all transactions that
originate from a logical call agent, as defined in Section 4.
Gateways can simply detect duplicate transactions by looking at the
transaction identifier only.
The Response Acknowledgement Attribute can be found in any command.
It carries a set of "confirmed transaction-id ranges" for final
responses received - provisional responses MUST NOT be confirmed. A
given response SHOULD NOT be confirmed in two separate messages.
MGCP entities MAY choose to delete the copies of the responses (but
not the transaction-id) to transactions whose id is included in
"confirmed transaction-id ranges" received in the Response
Confirmation messages (command or response). They SHOULD then
silently discard further commands from that entity when the
transaction-id falls within these ranges, and the response was issued
less than T-HIST seconds ago.
Entities MUST exercise due caution when acknowledging responses. In
particular, a response SHOULD only be acknowledged if the response
acknowledgement is sent to the same entity as the corresponding
command (i.e., the command whose response is being acknowledged) was
sent to.
Likewise, entities SHOULD NOT blindly accept a response
acknowledgement for a given response. However it is considered safe
to accept a response acknowledgement for a given response, when that
response acknowledgement is sent by the same entity as the command
that generated that response.
It should be noted, that use of response acknowledgments in commands
(as opposed to the Response Acknowledgement response following a
provisional response) is OPTIONAL. The benefit of using it is that
it reduces overall memory consumption. However, in order to avoid
large messages, implementations SHOULD NOT generate large response
acknowledgement lists. One strategy is to manage responses to
commands on a per endpoint basis. A command for an endpoint can
confirm a response to an older command for that same endpoint.
Responses to commands with wildcarded endpoint names can be confirmed
selectively with due consideration to message sizes, or alternatively
simply not be acknowledged (unless the response explicitly required a
Response Acknowledgement). Care must be taken to not confirm the
same response twice or a response that is more than T-HIST seconds
old.
The "confirmed transaction-id ranges" values SHALL NOT be used if
more than T-HIST seconds have elapsed since the entity issued its
last response to the other entity, or when an entity resumes
operation. In this situation, commands MUST be accepted and
processed, without any test on the transaction-id.
Commands that carry the "Response Acknowledgement attribute" may be
transmitted in disorder. The union of the "confirmed transaction-id
ranges" received in recent messages SHALL be retained.
3.5.3 Computing Retransmission Timers
It is the responsibility of the requesting entity to provide suitable
time outs for all outstanding commands, and to retry commands when
time outs have been exceeded. Furthermore, when repeated commands
fail to be acknowledged, it is the responsibility of the requesting
entity to seek redundant services and/or clear existing or pending
associations.
The specification purposely avoids specifying any value for the
retransmission timers. These values are typically network dependent.
The retransmission timers SHOULD normally estimate the timer by
measuring the time spent between the sending of a command and the
return of the first response to the command. At a minimum, a
retransmission strategy involving exponential bacKOFf MUST be
implemented. One possibility is to use the algorithm implemented in
TCP/IP, which uses two variables:
* the average acknowledgement delay, AAD, estimated through an
exponentially smoothed average of the observed delays,
* the average deviation, ADEV, estimated through an exponentially
smoothed average of the absolute value of the difference between
the observed delay and the current average.
The retransmission timer, RTO, in TCP, is set to the sum of the
average delay plus N times the average deviation, where N is a
constant. In MGCP, the maximum value of the timer SHOULD however be
bounded, in order to guarantee that no repeated packet will be
received by the gateways after T-HIST seconds. A suggested maximum
value for RTO (RTO-MAX) is 4 seconds. Implementers SHOULD consider
bounding the minimum value of this timer as well [19].
After any retransmission, the MGCP entity SHOULD do the following:
* It should double the estimated value of the acknowledgement delay
for this transaction, T-DELAY.
* It should compute a random value, uniformly distributed between 0.5
T-DELAY and T-DELAY.
* It should set the retransmission timer (RTO) to the minimum of:
- the sum of that random value and N times the average deviation,
- RTO-MAX.
This procedure has two effects. Because it includes an exponentially
increasing component, it will automatically slow down the stream of
messages in case of congestion. Because it includes a random
component, it will break the potential synchronization between
notifications triggered by the same external event.
Note that the estimators AAD and ADEV SHOULD NOT be updated for
transactions that involve retransmissions. Also, the first new
transmission following a successful retransmission SHOULD use the RTO
for that last retransmission. If this transmission succeeds without
any retransmissions, the AAD and ADEV estimators are updated and RTO
is determined as usual again. See, e.g., [18] for further details.
3.5.4 Maximum Datagram Size, Fragmentation and Reassembly
MGCP messages being transmitted over UDP rely on IP for fragmentation
and reassembly of large datagrams. The maximum theoretical size of
an IP datagram is 65535 bytes. With a 20-byte IP header and an 8-
byte UDP header, this leaves us with a maximum theoretical MGCP
message size of 65507 bytes when using UDP.
However, IP does not require a host to receive IP datagrams larger
than 576 bytes [21], which would provide an unacceptably small MGCP
message size. Consequently, MGCP mandates that implementations MUST
support MGCP datagrams up to at least 4000 bytes, which requires the
corresponding IP fragmentation and reassembly to be supported. Note,
that the 4000 byte limit applies to the MGCP level. Lower layer
overhead will require support for IP datagrams that are larger than
this: UDP and IP overhead will be at least 28 bytes, and, e.g., use
of IPSec will add additional overhead.
It should be noted, that the above applies to both Call Agents and
endpoints. Call Agents can audit endpoints to determine if they
support larger MGCP datagrams than specified above. Endpoints do
currently not have a similar capability to determine if a Call Agent
supports larger MGCP datagram sizes.
3.5.5 Piggybacking
There are cases when a Call Agent will want to send several messages
at the same time to the same gateways, and vice versa. When several
MGCP messages have to be sent in the same datagram, they MUST be
separated by a line of text that contains a single dot, as in for
example:
200 2005 OK
.
DLCX 1244 card23/21@tgw-7.example.net MGCP 1.0
C: A3C47F21456789F0
I: FDE234C8
The piggybacked messages MUST be processed exactly as if they had
been received one at a time in several separate datagrams. Each
message in the datagram MUST be processed to completion and in order
starting with the first message, and each command MUST be responded
to. Errors encountered in a message that was piggybacked MUST NOT
affect any of the other messages received in that datagram - each
message is processed on its own.
Piggybacking can be used to achieve two things:
* Guaranteed in-order delivery and processing of messages.
* Fate sharing of message delivery.
When piggybacking is used to guarantee in-order delivery of messages,
entities MUST ensure that this in-order delivery property is retained
on retransmissions of the individual messages. An example of this is
when multiple Notify's are sent using piggybacking (as described in
Section 4.4.1).
Fate sharing of message delivery ensures that either all the messages
are delivered, or none of them are delivered. When piggybacking is
used to guarantee this fate-sharing, entities MUST also ensure that
this property is retained upon retransmission. For example, upon
receiving a Notify from an endpoint operating in lockstep mode, the
Call Agent may wish to send the response and a new
NotificationRequest command in a single datagram to ensure message
delivery fate-sharing of the two.
3.5.6 Provisional Responses
Executing some transactions may require a long time. Long execution
times may interact with the timer based retransmission procedure.
This may result either in an inordinate number of retransmissions, or
in timer values that become too long to be efficient.
Gateways (and Call Agents) that can predict that a transaction will
require a long execution time SHOULD send a provisional response with
response code 100. As a guideline, a transaction that requires
external communication to complete, e.g., network resource
reservation, SHOULD issue a provisional response. Furthermore
entities SHOULD send a provisional response if they receive a
repetition of a transaction that has not yet finished executing.
Gateways (or Call Agents) that start building up queues of
transactions to be executed may send a provisional response with
response code 101 to indicate this (see Section 4.4.8 for further
details).
Pure transactional semantics would imply, that provisional responses
SHOULD NOT return any other information than the fact that the
transaction is currently executing, however an optimistic approach
allowing some information to be returned enables a reduction in the
delay that would otherwise be incurred in the system.
In order to reduce the delay in the system, it is RECOMMENDED to
include a connection identifier and session description in a 100
provisional response to the CreateConnection command. If a session
description would be returned by the ModifyConnection command, the
session description SHOULD be included in the provisional response
here as well. If the transaction completes successfully, the
information returned in the provisional response MUST be repeated in
the final response. It is considered a protocol error not to repeat
this information or to change any of the previously supplied
information in a successful response. If the transaction fails, an
error code is returned - the information returned previously is no
longer valid.
A currently executing CreateConnection or ModifyConnection
transaction MUST be cancelled if a DeleteConnection command for the
endpoint is received. In that case, a final response for the
cancelled transaction SHOULD still be returned automatically (error
code 407 - transaction aborted, is RECOMMENDED), and a final response
for the cancelled transaction MUST be returned if a retransmission of
the cancelled transaction is detected (see also Section 4.4.4).
MGCP entities that receive a provisional response SHALL switch to a
longer repetition timer (LONGTRAN-TIMER) for that transaction. The
purpose of this timer is primarily to detect processing failures.
The default value of LONGTRAN-TIMER is 5 seconds, however the
provisioning process may alter this. Note, that retransmissions MUST
still satisfy the timing requirements specified in Section 3.5.1 and
3.5.3. Consequently LONGTRAN-TIMER MUST be smaller than T-HIST (it
should in fact be considerably smaller). Also, entities MUST NOT let
a transaction run forever. A transaction that is timed out by the
entity SHOULD return error code 406 (transaction time-out). Per the
definition of T-HIST (Section 3.5.1), the maximum transaction
execution time is smaller than T-HIST (in a network with low delay,
it can reasonably safely be approximated as T-HIST minus T-MAX), and
a final response should be received no more than T-HIST seconds after
the command was sent initially. Nevertheless, entities SHOULD wait
for 2*T-HIST seconds before giving up on receiving a final response.
Retransmission of the command MUST still cease after T-MAX seconds
though. If a response is not received, the outcome of the
transaction is not known. If the entity sending the command was a
gateway, it now becomes "disconnected" and SHALL initiate the
"disconnected" procedure (see Section 4.4.7).
When the transaction finishes execution, the final response is sent
and the by now obsolete provisional response is deleted. In order to
ensure rapid detection of a lost final response, final responses
issued after provisional responses for a transaction SHOULD be
acknowledged (unfortunately older RFC2705 implementations may not do
this, which is the only reason it is not an absolute requirement).
The endpoint SHOULD therefore include an empty "ResponseAck"
parameter in those, and only those, final responses. The presence of
the "ResponseAck" parameter in the final response SHOULD trigger a
"Response Acknowledgement" response to be sent back to the endpoint.
The Response Acknowledgement" response will then include the
transaction-id of the response it acknowledges in the response
header. Note that, for backwards compatibility, entities cannot
depend on receiving such a "response acknowledgement", however it is
strongly RECOMMENDED to support this behavior, as excessive delays in
case of packet loss as well as excessive retransmissions may occur
otherwise.
Receipt of a "Response Acknowledgement" response is subject to the
same time-out and retransmission strategies and procedures as
responses to commands, i.e., the sender of the final response will
retransmit it if a "Response Acknowledgement" is not received in
time. For backwards compatibility, failure to receive a "response
acknowledgement" SHOULD NOT affect the roundtrip time estimates for
subsequent commands, and furthermore MUST NOT lead to the endpoint
becoming "disconnected". The "Response Acknowledgment" response is
never acknowledged.
4. States, Failover and Race Conditions
In order to implement proper call signaling, the Call Agent must keep
track of the state of the endpoint, and the gateway must make sure
that events are properly notified to the Call Agent. Special
conditions exist when the gateway or the Call Agent are restarted:
the gateway must be redirected to a new Call Agent during "failover"
procedures, the Call Agent must take special action when the gateway
is taken offline, or restarted.
4.1 Failover Assumptions and Highlights
The following protocol highlights are important to understanding Call
Agent fail-over mechanisms:
* Call Agents are identified by their domain name (and optional
port), not their network addresses, and several addresses can be
associated with a domain name.
* An endpoint has one and only one Call Agent associated with it at
any given point in time. The Call Agent associated with an
endpoint is the current value of the "notified entity". The
"notified entity" determines where the gateway will send it's
commands. If the "notified entity" does not include a port number,
the default Call Agent port number (2727) is assumed.
* NotifiedEntity is a parameter sent by the Call Agent to the gateway
to set the "notified entity" for the endpoint.
* The "notified entity" for an endpoint is the last value of the
NotifiedEntity parameter received for this endpoint. If no
explicit NotifiedEntity parameter has ever been received, the
"notified entity" defaults to a provisioned value. If no value was
provisioned or an empty NotifiedEntity parameter was provided (both
strongly discouraged) thereby making the "notified entity" empty,
the "notified entity" is set to the source address of the last
non-audit command for the endpoint. Thus auditing will not change
the "notified entity".
* Responses to commands are sent to the source address of the
command, regardless of the current "notified entity". When a
Notify message needs to be piggybacked with the response, the
datagram is still sent to the source address of the new command
received, regardless of the current "notified entity".
The ability for the "notified entity" to resolve to multiple network
addresses, allows a "notified entity" to represent a Call Agent with
multiple physical interfaces on it and/or a logical Call Agent made
up of multiple physical systems. The order of network addresses when
a DNS name resolves to multiple addresses is non-deterministic so
Call Agent fail-over schemes MUST NOT depend on any order (e.g., a
gateway MUST be able to send a "Notify" to any of the resolved
network addresses). On the other hand, the system is likely to be
most efficient if the gateway sends commands to the interface with
which it already has a current association. It is RECOMMENDED that
gateways use the following algorithm to achieve that goal:
* If the "notified entity" resolves to multiple network addresses,
and the source address of the request is one of those addresses,
that network address is the preferred destination address for
commands.
* If on the other hand, the source address of the request is not one
of the resolved addresses, the gateway must choose one of the
resolved addresses for commands.
* If the gateway fails to contact the network address chosen, it MUST
try the alternatives in the resolved list as described in Section
4.3.
If an entire Call Agent becomes unavailable, the endpoints managed by
that Call Agent will eventually become "disconnected". The only way
for these endpoints to become connected again is either for the
failed Call Agent to become available, or for a backup call agent to
contact the affected endpoints with a new "notified entity".
When a backup Call Agent has taken over control of a group of
endpoints, it is assumed that the failed Call Agent will communicate
and synchronize with the backup Call Agent in order to transfer
control of the affected endpoints back to the original Call Agent.
Alternatively, the failed Call Agent could simply become the backup
Call Agent.
We should note that handover conflict resolution between separate
CA's is not in place - we are relying strictly on the CA's knowing
what they are doing and communicating with each other (although
AuditEndpoint can be used to learn about the current "notified
entity"). If this is not the case, unexpected behavior may occur.
Note that as mentioned earlier, the default "notified entity" is
provisioned and may include both domain name and port. For small
gateways, provisioning may be done on a per endpoint basis. For much
larger gateways, a single provisioning element may be provided for
multiple endpoints or even for the entire gateway itself. In either
case, once the gateway powers up, each endpoint MUST have its own
"notified entity", so provisioned values for an aggregation of
endpoints MUST be copied to the "notified entity" for each endpoint
in the aggregation before operation proceeds. Where possible, the
RestartInProgress command on restart SHOULD be sent to the
provisioned "notified entity" based on an aggregation that allows the
"all of" wild-card to be used. This will reduce the number of
RestartInProgress messages.
Another way of viewing the use of "notified entity" is in terms of
associations between gateways and Call Agents. The "notified entity"
is a means to set up that association, and governs where the gateway
will send commands to. Commands received by the gateway however may
come from any source. The association is initially provisioned with
a provisioned "notified entity", so that on power up
RestartInProgress and persistent events that occur prior to the first
NotificationRequest from Call Agents will be sent to the provisioned
Call Agent. Once a Call Agent makes a request, however it may
include the NotifiedEntity parameter and set up a new association.
Since the "notified entity" persists across calls, the association
remains intact until a new "notified entity" is provided.
4.2 Communicating with Gateways
Endpoint names in gateways include a local name indicating the
specific endpoint and a domain name indicating the host/gateway where
the endpoint resides. Gateways may have several interfaces for
redundancy.
In gateways that have routing capability, the domain name may resolve
to a single network address with internal routing to that address
from any of the gateway's interfaces. In others, the domain name may
resolve to multiple network addresses, one for each interface. In
the latter case, if a Call Agent fails to contact the gateway on one
of the addresses, it MUST try the alternates.
4.3 Retransmission, and Detection of Lost Associations:
The media gateway control protocol is organized as a set of
transactions, each of which is composed of a command and a response,
commonly referred to as an acknowledgement. The MGCP messages, being
carried over UDP, may be subject to losses. In the absence of a
timely response, commands are retransmitted. MGCP entities MUST keep
in memory a list of the responses that they sent to recent
transactions, i.e., a list of all the responses they sent over the
last T-HIST seconds, and a list of the transactions that have not yet
finished executing.
The transaction identifiers of incoming commands are compared to the
transaction identifiers of the recent responses. If a match is
found, the MGCP entity does not execute the transaction, but simply
repeats the response. If a match to a previously responded to
transaction is not found, the transaction identifier of the incoming
command is compared to the list of transactions that have not yet
finished executing. If a match is found, the MGCP entity does not
execute the transaction again, but SHOULD simply send a provisional
response - a final response will be provided when the execution of
the command is complete (see Section 3.5.6 for further detail).
The repetition mechanism is used to guard against four types of
possible errors:
* transmission errors, when for example a packet is lost due to noise
on a line or congestion in a queue,
* component failure, when for example an interface to a Call Agent
becomes unavailable,
* Call Agent failure, when for example an entire Call Agent becomes
unavailable,
* failover, when a new Call Agent is "taking over" transparently.
The elements should be able to derive from the past history an
estimate of the packet loss rate due to transmission errors. In a
properly configured system, this loss rate should be very low,
typically less than 1%. If a Call Agent or a gateway has to repeat a
message more than a few times, it is very legitimate to assume that
something other than a transmission error is occurring. For example,
given a loss rate of 1%, the probability that 5 consecutive
transmission attempts fail is 1 in 100 billion, an event that should
occur less than once every 10 days for a Call Agent that processes
1,000 transactions per second. (Indeed, the number of
retransmissions that is considered excessive should be a function of
the prevailing packet loss rate.) We should note that the "suspicion
threshold", which we will call "Max1", is normally lower than the
"disconnection threshold", which we will call "Max2". Max2 MUST be
set to a larger value than Max1.
The MGCP retransmission algorithm is illustrated in the Figure below
and explained further in the following:
Command issued: N=0, T=0
+------------ retransmission: N++ <--------------+
if T <= T-Max then
transmission
+-- to new address, <-+<------------------------+
N=0
V V V
+-----------+
+--> awaiting - new Call Agent ->+ +------------+
response --- timer elapsed ---> T > T-Max ?
+-----------+ +------------+ ^ ^
v +-----(yes)-----+ (no)
(response
received) +------------+
N >= Max1 ?-(no)>+
v +------------+ ^ ^
+--------+
+<(no)- final ? (yes)
^ +--------+
(if first address & N=Max1,
v or last address & N=Max2
(yes) check DNS)
v V +---------------+
(end) more addresses?(yes)-->+
+---------------+
^
(no)
+------------+
N >= Max2 ?(no)--+
+------------+
(yes)
+----------------+
+-----------> T >= 2*T-HIST ?
+----------------+
(no) (yes)
+---------------<-----------------------+
v
(disconnected)
A classic retransmission algorithm would simply count the number of
successive repetitions, and conclude that the association is broken
after re-transmitting the packet an excessive number of times
(typically between 7 and 11 times). In order to account for the
possibility of an undetected or in-progress "failover", we modify the
classic algorithm as follows:
* We require that the gateway always checks for the presence of a new
Call Agent. It can be noticed either by:
- receiving a command where the NotifiedEntity points to the new
Call Agent, or
- receiving a redirection response pointing to a new Call Agent.
If a new Call Agent is detected, the gateway MUST start
retransmitting outstanding commands for the endpoint(s) redirected
to that new Call Agent. Responses to new or old commands are still
transmitted to the source address of the command.
* Prior to any retransmission, it is checked that the time elapsed
since the sending of the initial datagram is no greater than T-MAX.
If more than T-MAX time has elapsed, then retransmissions MUST
cease. If more than 2*T-HIST has elapsed, then the endpoint
becomes disconnected.
* If the number of repetitions for this Call Agent is equal to
"Max1", and its domain name was not resolved recently (e.g., within
the last 5 seconds or otherwise provisioned), and it is not in the
process of being resolved, then the gateway MAY actively query the
domain name server in order to detect the possible change of the
Call Agent interfaces. Note that the first repetition is the
second transmission.
* The gateway may have learned several IP addresses for the call
agent. If the number of repetitions for this IP address is greater
than or equal to "Max1" and lower than "Max2", and there are more
addresses that have not been tried, then the gateway MUST direct
the retransmissions to alternate addresses. Also, receipt of
explicit network notifications such as, e.g., ICMP network, host,
protocol, or port unreachable SHOULD lead the gateway to try
alternate addresses (with due consideration to possible security
issues).
* If there are no more interfaces to try, and the number of
repetitions for this address is Max2, then the gateway SHOULD
contact the DNS one more time to see if any other interfaces have
become available, unless the domain name was resolved recently
(e.g., within the last 5 seconds or otherwise provisioned), or it
is already in the process of being resolved. If there still are no
more interfaces to try, the gateway is then disconnected and MUST
initiate the "disconnected" procedure (see Section 4.4.7).
In order to automatically adapt to network load, MGCP specifies
exponentially increasing timers. If the initial timer is set to 200
milliseconds, the loss of a fifth retransmission will be detected
after about 6 seconds. This is probably an acceptable waiting delay
to detect a failover. The repetitions should continue after that
delay not only in order to perhaps overcome a transient connectivity
problem, but also in order to allow some more time for the execution
of a failover - waiting a total delay of 30 seconds is probably
acceptable.
It is however important that the maximum delay of retransmissions be
bounded. Prior to any retransmission, it is checked that the time
(T) elapsed since the sending of the initial datagram is no greater
than T-MAX. If more than T-MAX time has elapsed, retransmissions
MUST cease. If more than 2*T-HIST time has elapsed, the endpoint
becomes disconnected. The value T-MAX is related to the T-HIST
value: the T-HIST value MUST be greater than or equal to T-MAX plus
the maximum propagation delay in the network.
The default value for T-MAX is 20 seconds. Thus, if the assumed
maximum propagation delay is 10 seconds, then responses to old
transactions would have to be kept for a period of at least 30
seconds. The importance of having the sender and receiver agree on
these values cannot be overstated.
The default value for Max1 is 5 retransmissions and the default value
for Max2 is 7 retransmissions. Both of these values may be altered
by the provisioning process.
The provisioning process MUST be able to disable one or both of the
Max1 and Max2 DNS queries.
4.4 Race Conditions
MGCP deals with race conditions through the notion of a "quarantine
list" and through explicit detection of desynchronization, e.g., for
mismatched hook state due to glare for an endpoint.
MGCP does not assume that the transport mechanism will maintain the
order of commands and responses. This may cause race conditions,
that may be obviated through a proper behavior of the Call Agent.
(Note that some race conditions are inherent to distributed systems;
they would still occur, even if the commands were transmitted in
strict order.)
In some cases, many gateways may decide to restart operation at the
same time. This may occur, for example, if an area loses power or
transmission capability during an earthquake or an ice storm. When
power and transmission are reestablished, many gateways may decide to
send "RestartInProgress" commands simultaneously, leading to very
unstable operation.
4.4.1 Quarantine List
MGCP controlled gateways will receive "notification requests" that
ask them to watch for a list of "events". The protocol elements that
determine the handling of these events are the "Requested Events"
list, the "Digit Map", the "Quarantine Handling", and the "Detect
Events" list.
When the endpoint is initialized, the requested events list only
consists of persistent events for the endpoint, and the digit map is
assumed empty. At this point, the endpoint MAY use an implicit
NotificationRequest with the reserved RequestIdentifier zero ("0") to
detect and report a persistent event, e.g., off-hook. A pre-existing
off-hook condition MUST here result in the off-hook event being
generated as well.
The endpoint awaits the reception of a NotificationRequest command,
after which the gateway starts observing the endpoint for occurrences
of the events mentioned in the list, including persistent events.
The events are examined as they occur. The action that follows is
determined by the "action" parameter associated with the event in the
list of requested events, and also by the digit map. The events that
are defined as "accumulate" or "accumulate according to digit map"
are accumulated in a list of events, the events that are marked as
"accumulate according to the digit map" will additionally be
accumulated in the "current dial string". This will go on until one
event is encountered that triggers a notification which will be sent
to the current "notified entity".
The gateway, at this point, will transmit the Notify command and will
place the endpoint in a "notification" state. As long as the
endpoint is in this notification state, the events that are to be
detected on the endpoint are stored in a "quarantine" buffer (FIFO)
for later processing. The events are, in a sense, "quarantined".
All events that are specified by the union of the RequestedEvents
parameter and the most recently received DetectEvents parameter or,
in the absence of the latter, all events that are referred to in the
RequestedEvents, SHALL be detected and quarantined, regardless of the
action associated with the event. Persistent events are here viewed
as implicitly included in RequestedEvents. If the quarantine buffer
reaches the capacity of the endpoint, a Quarantine Buffer Overflow
event (see Appendix B) SHOULD be generated (when this event is
supported, the endpoint MUST ensure it has capacity to include the
event in the quarantine buffer). Excess events will now be
discarded.
The endpoint exits the "notification state" when the response
(whether success or failure) to the Notify command is received. The
Notify command may be retransmitted in the "notification state", as
specified in Section 3.5 and 4. If the endpoint is or becomes
disconnected (see Section 4.3) during this, a response to the Notify
command will never be received. The Notify command is then lost and
hence no longer considered pending, yet the endpoint is still in the
"notification state". Should that occur, completion of the
disconnected procedure specified in Section 4.4.7 SHALL then lead the
endpoint to exit the "notification state".
When the endpoint exits the "notification state" it resets the list
of observed events and the "current dial string" of the endpoint to a
null value.
Following that point, the behavior of the gateway depends on the
value of the QuarantineHandling parameter in the triggering
NotificationRequest command:
If the Call Agent had specified, that it expected at most one
notification in response to the notification request command, then
the gateway SHALL simply keep on accumulating events in the
quarantine buffer until it receives the next notification request
command.
If, however, the gateway is authorized to send multiple successive
Notify commands, it will proceed as follows. When the gateway exits
the "notification state", it resets the list of observed events and
the "current dial string" of the endpoint to a null value and starts
processing the list of quarantined events, using the already received
list of requested events and digit map. When processing these
events, the gateway may encounter an event which triggers a Notify
command to be sent. If that is the case, the gateway can adopt one
of the two following behaviors:
* it can immediately transmit a Notify command that will report all
events that were accumulated in the list of observed events until
the triggering event, included, leaving the unprocessed events in
the quarantine buffer,
* or it can attempt to empty the quarantine buffer and transmit a
single Notify command reporting several sets of events (in a single
list of observed events) and possibly several dial strings. The
"current dial string" is reset to a null value after each
triggering event. The events that follow the last triggering event
are left in the quarantine buffer.
If the gateway transmits a Notify command, the endpoint will reenter
and remain in the "notification state" until the acknowledgement is
received (as described above). If the gateway does not find a
quarantined event that triggers a Notify command, it places the
endpoint in a normal state. Events are then processed as they come,
in exactly the same way as if a Notification Request command had just
been received.
A gateway may receive at any time a new Notification Request command
for the endpoint, including the case where the endpoint is
disconnected. Activating an embedded Notification Request is here
viewed as receiving a new Notification Request as well, except that
the current list of ObservedEvents remains unmodified rather than
being processed again. When a new notification request is received
in the notification state, the gateway SHALL ensure that the pending
Notify is received by the Call Agent prior to a new Notify (note that
a Notify that was lost due to being disconnected, is no longer
considered pending). It does so by using the "piggybacking"
functionality of the protocol. The messages will then be sent in a
single packet to the current "notified entity". The steps involved
are the following:
a) the gateway sends a response to the new notification request.
b) the endpoint is then taken out of the "notification state" without
waiting for the acknowledgement of the pending Notify command.
c) a copy of the unacknowledged Notify command is kept until an
acknowledgement is received. If a timer elapses, the Notify will
be retransmitted.
d) If the gateway has to transmit a new Notify before the previous
Notify(s) is acknowledged, it constructs a packet that piggybacks
a repetition of the old Notify(s) and the new Notify (ordered by
age with the oldest first). This datagram will be sent to the
current "notified entity".
f) Gateways that cannot piggyback several messages in the same
datagram and hence guarantee in-order delivery of two (or more)
Notify's SHALL leave the endpoint in the "notification" state as
long as the last Notify is not acknowledged.
The procedure is illustrated by the following diagram:
+-------------------+
Processing Events <--------------------------------------+
+-------------------+
Need to send NTFY
v
+-------------------+
Outstanding NTFY ---- No -------+
+-------------------+ v
+-----------+
Yes Send NTFY
+-----------+
v
+--------------------+ v
Piggyback new NTFY +--------------------+
w. old outstanding ----> Notification State
NTFY(s) +--------------------+
+--------------------+
new RQNT NTFY response
received received
v
+-------------+
Step mode ? - No ->+
+-------------+ ^
Yes
v
+---------------+
Wait for RQNT
+---------------+
RQNT received
v
+---------------+
+------> Apply RQNT and----->+
send response
+---------------+
Gateways may also attempt to deliver the pending Notify prior to a
successful response to the new NotificationRequest by using the
"piggybacking" functionality of the protocol. This was in fact
required behavior in RFC2705, however there are several
complications in doing this, and the benefits are questionable. In
particular, the RFC2705 mechanism did not guarantee in-order
delivery of Notify's and responses to NotificationRequests in
general, and hence Call Agents had to handle out-of-order delivery of
these messages anyway. The change to optional status is thus
backwards compatible while greatly reducing complexity.
After receiving the Notification Request command, the requested
events list and digit map (if a new one was provided) are replaced by
the newly received parameters, and the current dial string is reset
to a null value. Furthermore, when the Notification Request was
received in the "notification state", the list of observed events is
reset to a null value. The subsequent behavior is conditioned by the
value of the QuarantineHandling parameter. The parameter may specify
that quarantined events (and observed events which in this case is
now an empty list), should be discarded, in which case they will be.
If the parameter specifies that the quarantined (and observed) events
are to be processed, the gateway will start processing the list of
quarantined (and observed) events, using the newly received list of
requested events and digit map (if provided). When processing these
events, the gateway may encounter an event which requires a Notify
command to be sent. If that is the case, the gateway will
immediately transmit a Notify command that will report all events
that were accumulated in the list of observed events until the
triggering event, included leaving the unprocessed events in the
quarantine buffer, and will enter the "notification state".
A new notification request may be received while the gateway has
accumulated events according to the previous notification request,
but has not yet detected a notification-triggering events, i.e., the
endpoint is not in the "notification state". The handling of not-
yet-notified events is determined, as with the quarantined events, by
the quarantine handling parameter:
* If the quarantine-handling parameter specifies that quarantined
events shall be ignored, the observed events list is simply reset.
* If the quarantine-handling parameter specifies that quarantined
events shall be processed, the observed event list is transferred
to the quarantined event list. The observed event list is then
reset, and the quarantined event list is processed.
Call Agents controlling endpoints in lockstep mode SHOULD provide the
response to a successful Notify message and the new
NotificationRequest in the same datagram using the piggybacking
mechanism.
4.4.2 Explicit Detection
A key element of the state of several endpoints is the position of
the hook. A race condition may occur when the user decides to go
off-hook before the Call Agent has the time to ask the gateway to
notify an off-hook event (the "glare" condition well known in
telephony), or if the user goes on-hook before the Call Agent has the
time to request the event's notification.
To avoid this race condition, the gateway MUST check the condition of
the endpoint before acknowledging a NotificationRequest. It MUST
return an error:
1. If the gateway is requested to notify an "off-hook" transition
while the phone is already off-hook, (error code 401 - phone off
hook)
2. If the gateway is requested to notify an "on-hook" or "flash hook"
condition while the phone is already on-hook (error code 402 -
phone on hook).
Additionally, individual signal definitions can specify that a signal
will only operate under certain conditions, e.g., ringing may only be
possible if the phone is already off-hook. If such prerequisites
exist for a given signal, the gateway MUST return the error specified
in the signal definition if the prerequisite is not met.
It should be noted, that the condition check is performed at the time
the notification request is received, whereas the actual event that
caused the current condition may have either been reported, or
ignored earlier, or it may currently be quarantined.
The other state variables of the gateway, such as the list of
RequestedEvents or list of requested signals, are entirely replaced
after each successful NotificationRequest, which prevents any long
term discrepancy between the Call Agent and the gateway.
When a NotificationRequest is unsuccessful, whether it is included in
a connection-handling command or not, the gateway MUST simply
continue as if the command had never been received. As all other
transactions, the NotificationRequest MUST operate as an atomic
transaction, thus any changes initiated as a result of the command
MUST be reverted.
Another race condition may occur when a Notify is issued shortly
before the reception by the gateway of a NotificationRequest. The
RequestIdentifier is used to correlate Notify commands with
NotificationRequest commands thereby enabling the Call Agent to
determine if the Notify command was generated before or after the
gateway received the new NotificationRequest. This is especially
important to avoid deadlocks in "step" mode.
4.4.3 Transactional Semantics
As the potential transaction completion times increase, e.g., due to
external resource reservations, a careful definition of the
transactional semantics becomes increasingly important. In
particular the issue of race conditions, e.g., as it relates to
hook-state, must be defined carefully.
An important point to consider is, that the status of a pre-condition
(e.g., hook-state) may in fact change between the time a transaction
starts and the time it either completes successfully (transaction
commit) or fails. In general, we can say that the successful
execution of a transaction depends on one or more pre-conditions
where the status of one or more of the pre-conditions may change
dynamically between the transaction start and transaction commit.
The simplest semantics for this is simply to require that all pre-
conditions be met from the time the transaction is initiated until
the transaction commits. If any pre-condition is not met before the
completion of the transaction, the transaction will also fail.
As an example, consider a transaction that includes a request for the
"off-hook" event. When the transaction is initiated the phone is
"on-hook" and this pre-condition is therefore met. If the hook-state
changes to "off-hook" before the transaction completes, the pre-
condition is no longer met, and the transaction therefore immediately
fails.
Finally, we need to consider the point in time when a new transaction
takes effect and endpoint processing according to an old transaction
stops. For example, assume that transaction T1 has been executed
successfully and event processing is currently being done according
to transaction T1. Now we receive a new transaction T2 specifying
new event processing (for example a CreateConnection with an
encapsulated NotificationRequest). Since we don't know whether T2
will complete successfully or not, we cannot start processing events
according to T2 until the outcome of T2 is known. While we could
suspend all event processing until the outcome of T2 is known, this
would make for a less responsive system and hence SHOULD NOT be done.
Instead, when a new transaction Ty is received and Ty modifies
processing according to an old transaction Tx, processing according
to Tx SHOULD remain active for as long as possible, until a
successful outcome of Ty is known to occur. If Ty fails, then
processing according to Tx will of course continue as usual. Any
changes incurred by Ty logically takes effect when Ty commits. Thus,
if the endpoint was in the notification state when Ty commits, and Ty
contained a NotificationRequest, the endpoint will be taken out of
the notification state when Ty commits. Note that this is
independent of whether the endpoint was in the notification state
when Ty was initiated. For example, a Notify could be generated due
to processing according to Tx between the start and commit of Ty. If
the commit of Ty leads to the endpoint entering the notification
state, a new NotificationRequest (Tz) is needed to exit the
notification state. This follows from the fact that transaction
execution respects causal order.
Another related issue is the use of wildcards, especially the "all
of" wildcard, which may match more than one endpoint. When a command
is requested, and the endpoint identifier matches more than one
endpoint, transactional semantics still apply. Thus, the command
MUST either succeed for all the endpoints, or it MUST fail for all of
them. A single response is consequently always issued.
4.4.4 Ordering of Commands, and Treatment of Misorder
MGCP does not mandate that the underlying transport protocol
guarantees in-order delivery of commands to a gateway or an endpoint.
This property tends to maximize the timeliness of actions, but it has
a few drawbacks. For example:
* Notify commands may be delayed and arrive at the Call Agent after
the transmission of a new Notification Request command,
* If a new NotificationRequest is transmitted before a previous one
is acknowledged, there is no guarantee that the previous one will
not be received and executed after the new one.
Call Agents that want to guarantee consistent operation of the
endpoints can use the following rules:
1) When a gateway handles several endpoints, commands pertaining to
the different endpoints can be sent in parallel, for example
following a model where each endpoint is controlled by its own
process or its own thread.
2) When several connections are created on the same endpoint,
commands pertaining to different connections can be sent in
parallel.
3) On a given connection, there should normally be only one
outstanding command (create or modify). However, a
DeleteConnection command can be issued at any time. In
consequence, a gateway may sometimes receive a ModifyConnection
command that applies to a previously deleted connection. Such
commands will fail, and an error code MUST be returned (error code
515 - incorrect connection-id, is RECOMMENDED).
4) On a given endpoint, there should normally be only one outstanding
NotificationRequest command at any time. The RequestId parameter
MUST be used to correlate Notify commands with the triggering
notification request.
5) In some cases, an implicitly or explicitly wildcarded
DeleteConnection command that applies to a group of endpoints can
step in front of a pending CreateConnection command. The Call
Agent should individually delete all connections whose completion
was pending at the time of the global DeleteConnection command.
Also, new CreateConnection commands for endpoints named by the
wild-carding SHOULD NOT be sent until the wild-carded
DeleteConnection command is acknowledged.
6) When commands are embedded within each other, sequencing
requirements for all commands must be adhered to. For example a
Create Connection command with a Notification Request in it must
adhere to the sequencing requirements associated with both
CreateConnection and NotificationRequest at the same time.
7) AuditEndpoint and AuditConnection are not subject to any
sequencing requirements.
8) RestartInProgress MUST always be the first command sent by an
endpoint as defined by the restart procedure. Any other command
or non-restart response (see Section 4.4.6), except for responses
to auditing, MUST be delivered after this RestartInProgress
command (piggybacking allowed).
9) When multiple messages are piggybacked in a single packet, the
messages are always processed in order.
10) On a given endpoint, there should normally be only one
outstanding EndpointConfiguration command at any time.
Gateways MUST NOT make any assumptions as to whether Call Agents
follow these rules or not. Consequently gateways MUST always respond
to commands, regardless of whether they adhere to the above rules or
not. To ensure consistent operation, gateways SHOULD behave as
specified below when one or more of the above rules are not followed:
* Where a single outstanding command is expected (ModifyConnection,
NotificationRequest, and EndpointConfiguration), but the same
command is received in a new transaction before the old finishes
executing, the gateway SHOULD fail the previous command. This
includes the case where one or more of the commands were
encapsulated. The use of error code 407 (transaction aborted) is
RECOMMENDED.
* If a ModifyConnection command is received for a pending
CreateConnection command, the ModifyConnection command SHOULD
simply be rejected. The use of error code 400 (transient error) is
RECOMMENDED. Note that this situation constitutes a Call Agent
programming error.
* If a DeleteConnection command is received for a pending
CreateConnection or ModifyConnection command, the pending command
MUST be aborted. The use of error code 407 (transaction aborted)
is RECOMMENDED.
Note, that where reception of a new command leads to aborting an old
command, the old command SHOULD be aborted regardless of whether the
new command succeeds or not. For example, if a ModifyConnection
command is aborted by a DeleteConnection command which itself fails
due to an encapsulated NotificationRequest, the ModifyConnection
command is still aborted.
4.4.5 Endpoint Service States
As described earlier, endpoints configured for operation may be
either in-service or out-of-service. The actual service-state of the
endpoint is reflected by the combination of the RestartMethod and
RestartDelay parameters, which are sent with RestartInProgress
commands (Section 2.3.12) and furthermore may be audited in
AuditEndpoint commands (Section 2.3.10).
The service-state of an endpoint affects how it processes a command.
An endpoint in-service MUST process any command received, whereas an
endpoint that is out-of-service MUST reject non-auditing commands,
but SHOULD process auditing commands if possible. For backwards
compatibility, auditing commands for an out-of-service endpoint may
alternatively be rejected as well. Any command rejected due to an
endpoint being out-of-service SHOULD generate error code 501
(endpoint not ready/out-of-service).
Note that (per Section 2.1.2), unless otherwise specified for a
command, endpoint names containing the "any of" wildcard only refer
to endpoints in-service, whereas endpoint names containing the "all
of" wildcard refer to all endpoints, regardless of service state.
The above relationships are illustrated in the table below which
shows the current service-states and gateway processing of commands
as a function of the RestartInProgress command sent and the response
(if any) received to it. The last column also lists (in parentheses)
the RestartMethod to be returned if audited:
------------------------------------------------------------------
Restart- Restart- 2xx Service- Response to
Method Delay received ? State new command
------------------------------------------------------------------
graceful zero Yes/No In non-audit: 2xx
audit: 2xx
(graceful)
-----------+----------+-----------+----------+--------------------
graceful non-zero Yes/No In* non-audit: 2xx
audit: 2xx
(graceful)
-----------+----------+-----------+----------+--------------------
forced N/A Yes/No Out non-audit: 501
audit: 2xx
(forced)
-----------+----------+-----------+----------+--------------------
restart zero No In non-audit: 2xx,405*
audit: 2xx
(restart)
-----------+----------+-----------+----------+--------------------
restart zero Yes In non-audit: 2xx
audit: 2xx
(restart)
-----------+----------+-----------+----------+--------------------
restart non-zero No Out* non-audit: 501*
audit: 2xx
(restart)
-----------+----------+-----------+----------+--------------------
restart non-zero Yes Out* non-audit: 501*
audit: 2xx
(restart)
-----------+----------+-----------+----------+--------------------
discon- zero/ No In non-audit: 2xx,
nected non-zero audit: 2xx
(disconnected)
-----------+----------+-----------+----------+--------------------
discon- zero/ Yes In non-audit: 2xx
nected non-zero audit: 2xx
(restart)
-----------+----------+-----------+----------+--------------------
cancel- N/A Yes/No In non-audit: 2xx
graceful audit: 2xx
(restart)
------------------------------------------------------------------
Notes (*):
* The three service-states marked with "*" will change after the
expiration of the RestartDelay at which time an updated
RestartInProgress command SHOULD be sent.
* If the endpoint returns 2xx when the restart procedure has not yet
completed, then in-order delivery MUST still be satisfied, i.e.,
piggy-backing is to be used. If instead, the command is not
processed, 405 SHOULD be returned.
* Following a "restart" RestartInProgress with a non-zero
RestartDelay, error code 501 is only returned until the endpoint
goes in-service, i.e., until the expiration of the RestartDelay.
4.4.6 Fighting the Restart Avalanche
Let's suppose that a large number of gateways are powered on
simultaneously. If they were to all initiate a RestartInProgress
transaction, the Call Agent would very likely be swamped, leading to
message losses and network congestion during the critical period of
service restoration. In order to prevent such avalanches, the
following behavior is REQUIRED:
1) When a gateway is powered on, it MUST initiate a restart timer to
a random value, uniformly distributed between 0 and a maximum
waiting delay (MWD). Care should be taken to avoid synchronicity
of the random number generation between multiple gateways that
would use the same algorithm.
2) The gateway MUST then wait for either the end of this timer, the
reception of a command from the Call Agent, or the detection of a
local user activity, such as for example an off-hook transition on
a residential gateway.
3) When the timer elapses, when a command is received, or when an
activity is detected, the gateway MUST initiate the restart
procedure.
The restart procedure simply requires the endpoint to guarantee that
the first
* non-audit command, or
* non-restart response (i.e., error codes other than 405, 501, and
520) to a non-audit command
that the Call Agent sees from this endpoint is a "restart"
RestartInProgress command. The endpoint is free to take full
advantage of piggybacking to achieve this. Endpoints that are
considered in-service will have a RestartMethod of "restart", whereas
endpoints considered out-of-service will have a RestartMethod of
"forced" (also see Section 4.4.5). Commands rejected due to an
endpoint not yet having completed the restart procedure SHOULD use
error code 405 (endpoint "restarting").
The restart procedure is complete once a success response has been
received. If an error response is received, the subsequent behavior
depends on the error code in question:
* If the error code indicates a transient error (4xx), then the
restart procedure MUST be initiated again (as a new transaction).
* If the error code is 521, then the endpoint is redirected, and the
restart procedure MUST be initiated again (as a new transaction).
The 521 response MUST have included a NotifiedEntity which then is
the "notified entity" towards which the restart is initiated. If
it did not include a NotifiedEntity, the response is treated as any
other permanent error (see below).
* If the error is any other permanent error (5xx), and the endpoint
is not able to rectify the error, then the endpoint no longer
initiates the restart procedure on its own (until
rebooted/restarted) unless otherwise specified. If a command is
received for the endpoint, the endpoint MUST initiate the restart
procedure again.
Note that if the RestartInProgress is piggybacked with the response
(R) to a command received while restarting, then retransmission of
the RestartInProgress does not require piggybacking of the response
R. However, while the endpoint is restarting, a resend of the
response R does require the RestartInProgress to be piggybacked to
ensure in-order delivery of the two.
Should the gateway enter the "disconnected" state while carrying out
the restart procedure, the disconnected procedure specified in
Section 4.4.7 MUST be carried out, except that a "restart" rather
than "disconnected" message is sent during the procedure.
Each endpoint in a gateway will have a provisionable Call Agent,
i.e., "notified entity", to direct the initial restart message
towards. When the collection of endpoints in a gateway is managed by
more than one Call Agent, the above procedure MUST be performed for
each collection of endpoints managed by a given Call Agent. The
gateway MUST take full advantage of wild-carding to minimize the
number of RestartInProgress messages generated when multiple
endpoints in a gateway restart and the endpoints are managed by the
same Call Agent. Note that during startup, it is possible for
endpoints to start out as being out-of-service, and then become in-
service as part of the gateway initialization procedure. A gateway
may thus choose to send first a "forced" RestartInProgress for all
its endpoints, and subsequently a "restart" RestartInProgress for the
endpoints that come in-service. Alternatively, the gateway may
simply send "restart" RestartInProgress for only those endpoints that
are in-service, and "forced" RestartInProgress for the specific
endpoints that are out-of-service. Wild-carding MUST still be used
to minimize the number of messages sent though.
The value of MWD is a configuration parameter that depends on the
type of the gateway. The following reasoning can be used to
determine the value of this delay on residential gateways.
Call agents are typically dimensioned to handle the peak hour traffic
load, during which, in average, 10% of the lines will be busy,
placing calls whose average duration is typically 3 minutes. The
processing of a call typically involves 5 to 6 MGCP transactions
between each endpoint and the Call Agent. This simple calculation
shows that the Call Agent is expected to handle 5 to 6 transactions
for each endpoint, every 30 minutes on average, or, to put it
otherwise, about one transaction per endpoint every 5 to 6 minutes on
average. This suggest that a reasonable value of MWD for a
residential gateway would be 10 to 12 minutes. In the absence of
explicit configuration, residential gateways should adopt a value of
600 seconds for MWD.
The same reasoning suggests that the value of MWD should be much
shorter for trunking gateways or for business gateways, because they
handle a large number of endpoints, and also because the usage rate
of these endpoints is much higher than 10% during the peak busy hour,
a typical value being 60%. These endpoints, during the peak hour,
are thus expected to contribute about one transaction per minute to
the Call Agent load. A reasonable algorithm is to make the value of
MWD per "trunk" endpoint six times shorter than the MWD per
residential gateway, and also inversely proportional to the number of
endpoints that are being restarted. For example MWD should be set to
2.5 seconds for a gateway that handles a T1 line, or to 60
milliseconds for a gateway that handles a T3 line.
4.4.7 Disconnected Endpoints
In addition to the restart procedure, gateways also have a
"disconnected" procedure, which MUST be initiated when an endpoint
becomes "disconnected" as described in Section 4.3. It should here
be noted, that endpoints can only become disconnected when they
attempt to communicate with the Call Agent. The following steps MUST
be followed by an endpoint that becomes "disconnected":
1. A "disconnected" timer is initialized to a random value, uniformly
distributed between 1 and a provisionable "disconnected" initial
waiting delay (Tdinit), e.g., 15 seconds. Care MUST be taken to
avoid synchronicity of the random number generation between
multiple gateways and endpoints that would use the same algorithm.
2. The gateway then waits for either the end of this timer, the
reception of a command for the endpoint from the Call Agent, or
the detection of a local user activity for the endpoint, such as
for example an off-hook transition.
3. When the "disconnected" timer elapses for the endpoint, when a
command is received for the endpoint, or when local user activity
is detected for the endpoint, the gateway initiates the
"disconnected" procedure for the endpoint - if a disconnected
procedure was already in progress for the endpoint, it is simply
replaced by the new one. Furthermore, in the case of local user
activity, a provisionable "disconnected" minimum waiting delay
(Tdmin) MUST have elapsed since the endpoint became disconnected
or the last time it ended the "disconnected" procedure in order to
limit the rate at which the procedure is performed. If Tdmin has
not passed, the endpoint simply proceeds to step 2 again, without
affecting any disconnected procedure already in progress.
4. If the "disconnected" procedure still left the endpoint
disconnected, the "disconnected" timer is then doubled, subject to
a provisionable "disconnected" maximum waiting delay (Tdmax),
e.g., 600 seconds, and the gateway proceeds with step 2 again
(using a new transaction-id).
The "disconnected" procedure is similar to the restart procedure in
that it simply states that the endpoint MUST send a RestartInProgress
command to the Call Agent informing it that the endpoint was
disconnected. Furthermore, the endpoint MUST guarantee that the
first non-audit message (non-audit command or response to non-audit
command) that the Call Agent sees from this endpoint MUST inform the
Call Agent that the endpoint is disconnected (unless the endpoint
goes out-of-service). When a command (C) is received, this is
achieved by sending a piggy-backed datagram with a "disconnected"
RestartInProgress command and the response to command C to the source
address of command C as opposed to the current "notified entity".
This piggy-backed RestartInProgress is not automatically
retransmitted by the endpoint but simply relies on fate-sharing with
the piggy-backed response to guarantee the in-order delivery
requirement. The Call Agent still sends a response to the piggy-
backed RestartInProgress, however, as usual, the response may be
lost. In addition to the piggy-backed RestartInProgress command, a
new "disconnected" procedure is triggered by the command received.
This will lead to a non piggy-backed copy (i.e., same transaction) of
the "disconnected" RestartInProgress command being sent reliably to
the current "notified entity".
When the Call Agent learns that the endpoint is disconnected, the
Call Agent may then for instance decide to audit the endpoint, or
simply clear all connections for the endpoint. Note that each such
"disconnected" procedure will result in a new RestartInProgress
command, which will be subject to the normal retransmission
procedures specified in Section 4.3. At the end of the procedure,
the endpoint may thus still be "disconnected". Should the endpoint
go out-of-service while being disconnected, it SHOULD send a "forced"
RestartInProgress message as described in Section 2.3.12.
The disconnected procedure is complete once a success response has
been received. Error responses are handled similarly to the restart
procedure (Section 4.4.6). If the "disconnected" procedure is to be
initiated again following an error response, the rate-limiting timer
considerations specified above still apply.
Note, that if the RestartInProgress is piggybacked with the response
(R) to a command received while being disconnected, then
retransmission of this particular RestartInProgress does not require
piggybacking of the response R. However, while the endpoint is
disconnected, resending the response R does require the
RestartInProgress to be piggybacked with the response to ensure the
in-order delivery of the two.
If a set of disconnected endpoints have the same "notified entity",
and the set of endpoints can be named with a wildcard, the gateway
MAY replace the individual disconnected procedures with a suitably
wildcarded disconnected procedure instead. In that case, the Restart
Delay for the wildcarded "disconnected" RestartInProgress command
SHALL be the Restart Delay corresponding to the oldest disconnected
procedure replaced. Note that if only a subset of these endpoints
subsequently have their "notified entity" changed and/or are no
longer disconnected, then that wildcarded disconnected procedure can
no longer be used. The remaining individual disconnected procedures
MUST then be resumed again.
A disconnected endpoint may wish to send a command (besides
RestartInProgress) while it is disconnected. Doing so will only
succeed once the Call Agent is reachable again, which raises the
question of what to do with such a command meanwhile. At one
extreme, the endpoint could drop the command right away, however that
would not work very well when the Call Agent was in fact available,
but the endpoint had not yet completed the "disconnected" procedure
(consider for example the case where a NotificationRequest was just
received which immediately resulted in a Notify being generated). To
prevent such scenarios, disconnected endpoints SHALL NOT blindly drop
new commands to be sent for a period of T-MAX seconds after they
receive a non-audit command.
One way of satisfying this requirement is to employ a temporary
buffering of commands to be sent, however in doing so, the endpoint
MUST ensure, that it:
* does not build up a long queue of commands to be sent,
* does not swamp the Call Agent by rapidly sending too many commands
once it is connected again.
Buffering commands for T-MAX seconds and, once the endpoint is
connected again, limiting the rate at which buffered commands are
sent to one outstanding command per endpoint is considered acceptable
(see also Section 4.4.8, especially if using wildcards). If the
endpoint is not connected within T-MAX seconds, but a "disconnected"
procedure is initiated within T-MAX seconds, the endpoint MAY
piggyback the buffered command(s) with that RestartInProgress. Note,
that once a command has been sent, regardless of whether it was
buffered initially, or piggybacked earlier, retransmission of that
command MUST cease T-MAX seconds after the initial send as described
in Section 4.3.
This specification purposely does not specify any additional behavior
for a disconnected endpoint. Vendors MAY for instance choose to
provide silence, play reorder tone, or even enable a downloaded wav
file to be played.
The default value for Tdinit is 15 seconds, the default value for
Tdmin, is 15 seconds, and the default value for Tdmax is 600 seconds.
4.4.8 Load Control in General
The previous sections have described several MGCP mechanisms to deal
with congestion and overload, namely:
* the UDP retransmission strategy which adapts to network and call
agent congestion on a per endpoint basis,
* the guidelines on the ordering of commands which limit the number
of commands issued in parallel,
* the restart procedure which prevents flooding in case of a restart
avalanche, and
* the disconnected procedure which prevents flooding in case of a
large number of disconnected endpoints.
It is however still possible for a given set of endpoints, either on
the same or different gateways, to issue one or more commands at a
given point in time. Although it can be argued, that Call Agents
should be sized to handle one message per served endpoint at any
given point in time, this may not always be the case in practice.
Similarly, gateways may not be able to handle a message for all of
its endpoints at any given point in time. In general, such issues
can be dealt with through the use of a credit-based mechanism, or by
monitoring and automatically adapting to the observed behavior. We
opt for the latter approach as follows.
Conceptually, we assume that Call Agents and gateways maintain a
queue of incoming transactions to be executed. Associated with this
transaction queue is a high-water and a low-water mark. Once the
queue length reaches the high-water mark, the entity SHOULD start
issuing 101 provisional responses (transaction queued) until the
queue length drops to the low-water mark. This applies to new
transactions as well as to retransmissions. If the entity is unable
to process any new transactions at this time, it SHOULD return error
code 409 (processing overload).
Furthermore, gateways SHOULD adjust the sending rate of new commands
to a given Call Agent by monitoring the observed response times from
that Call Agent to a *set* of endpoints. If the observed smoothed
average response time suddenly rises significantly over some
threshold, or the gateway receives a 101 (transaction queued) or 409
(overload) response, the gateway SHOULD adjust the sending rate of
new commands to that Call Agent accordingly. The details of the
smoothing average algorithm, the rate adjustments, and the thresholds
involved are for further study, however they MUST be configurable.
Similarly, Call Agents SHOULD adjust the sending rate of new
transactions to a given gateway by monitoring the observed response
times from that gateway for a *set* of endpoints. If the observed
smoothed average response time suddenly rises significantly over some
threshold, or the Call Agent receives a 101 (transaction queued) or
409 (overloaded), the Call Agent SHOULD adjust the sending rate of
new commands to that gateway accordingly. The details of the
smoothing average algorithm, the rate adjustments, and the thresholds
involved are for further study, however they MUST be configurable.
5. Security Requirements
Any entity can send a command to an MGCP endpoint. If unauthorized
entities could use the MGCP, they would be able to set-up
unauthorized calls, or to interfere with authorized calls. We expect
that MGCP messages will always be carried over secure Internet
connections, as defined in the IP security architecture as defined in
RFC2401, using either the IP Authentication Header, defined in RFC
2402, or the IP Encapsulating Security Payload, defined in RFC2406.
The complete MGCP protocol stack would thus include the following
layers:
-------------------------------
MGCP
-------------------------------
UDP
-------------------------------
IP security
(authentication or encryption)
-------------------------------
IP
-------------------------------
transmission media
-------------------------------
Adequate protection of the connections will be achieved if the
gateways and the Call Agents only accept messages for which IP
security provided an authentication service. An encryption service
will provide additional protection against eavesdropping, thus
preventing third parties from monitoring the connections set up by a
given endpoint.
The encryption service will also be requested if the session
descriptions are used to carry session keys, as defined in SDP.
These procedures do not necessarily protect against denial of service
attacks by misbehaving gateways or misbehaving Call Agents. However,
they will provide an identification of these misbehaving entities,
which should then be deprived of their authorization through
maintenance procedures.
5.1 Protection of Media Connections
MGCP allows Call Agent to provide gateways with "session keys" that
can be used to encrypt the audio messages, protecting against
eavesdropping.
A specific problem of packet networks is "uncontrolled barge-in".
This attack can be performed by directing media packets to the IP
address and UDP port used by a connection. If no protection is
implemented, the packets will be decoded and the signals will be
played on the "line side".
A basic protection against this attack is to only accept packets from
known sources, however this tends to conflict with RTP principles.
This also has two inconveniences: it slows down connection
establishment and it can be fooled by source spoofing:
* To enable the address-based protection, the Call Agent must obtain
the source address of the egress gateway and pass it to the ingress
gateway. This requires at least one network round trip, and leaves
us with a dilemma: either allow the call to proceed without
waiting for the round trip to complete, and risk for example
"clipping" a remote announcement, or wait for the full round trip
and settle for slower call-set-up procedures.
* Source spoofing is only effective if the attacker can obtain valid
pairs of source and destination addresses and ports, for example by
listening to a fraction of the traffic. To fight source spoofing,
one could try to control all access points to the network. But
this is in practice very hard to achieve.
An alternative to checking the source address is to encrypt and
authenticate the packets, using a secret key that is conveyed during
the call set-up procedure. This will not slow down the call set-up,
and provides strong protection against address spoofing.
6. Packages
As described in Section 2.1.6, packages are the preferred way of
extending MGCP. In this section we describe the requirements
associated with defining a package.
A package MUST have a unique package name defined. The package name
MUST be registered with the IANA, unless it starts with the
characters "x-" or "x+" which are reserved for experimental packages.
Please refer to Appendix C for IANA considerations.
A package MUST also have a version defined which is simply a non-
negative integer. The default and initial version of a package is
zero, the next version is one, etc. New package versions MUST be
completely backwards compatible, i.e., a new version of a package
MUST NOT redefine or remove any of the extensions provided in an
earlier version of the package. If such a need arises, a new package
name MUST be used instead.
Packages containing signals of type time-out MAY indicate if the "to"
parameter is supported for all the time-out signals in the package as
well as the default rounding rules associated with these (see Section
3.2.2.4). If no such definition is provided, each time-out signal
SHOULD provide these definitions.
A package defines one or more of the following extensions:
* Actions
* BearerInformation
* ConnectionModes
* ConnectionParameters
* DigitMapLetters
* Events and Signals
* ExtensionParameters
* LocalConnectionOptions
* ReasonCodes
* RestartMethods
* Return codes
For each of the above types of extensions supported by the package,
the package definition MUST contain a description of the extension as
defined in the following sections. Please note, that package
extensions, just like any other extension, MUST adhere to the MGCP
grammar.
6.1 Actions
Extension Actions SHALL include:
* The name and encoding of the extension action.
* If the extension action takes any action parameters, then the name,
encoding, and possible values of those parameters.
* A description of the operation of the extension action.
* A listing of the actions in this specification the extension can be
combined with. If such a listing is not provided, it is assumed
that the extension action cannot be combined with any other action
in this specification.
* If more than one extension action is defined in the package, then a
listing of the actions in the package the extension can be combined
with. If such a listing is not provided, it is assumed that the
extension action cannot be combined with any other action in the
package.
Extension actions defined in two or more different packages SHOULD
NOT be used simultaneously, unless very careful consideration to
their potential interaction and side-effects has been given.
6.2 BearerInformation
BearerInformation extensions SHALL include:
* The name and encoding of the BearerInformation extension.
* The possible values and encoding of those values that can be
assigned to the BearerInformation extension.
* A description of the operation of the BearerInformation extension.
As part of this description the default value (if any) if the
extension is omitted in an EndpointConfiguration command MUST be
defined. It may be necessary to make a distinction between the
default value before and after the initial application of the
parameter, for example if the parameter retains its previous value
once specified, until explicitly altered. If default values are
not described, then the extension parameter simply defaults to
empty in all EndpointConfiguration commands.
Note that the extension SHALL be included in the result for an
AuditEndpoint command auditing the BearerInformation.
6.3 ConnectionModes
Extension Connection Modes SHALL include:
* The name and encoding of the extension connection mode.
* A description of the operation of the extension connection mode.
* A description of the interaction a connection in the extension
connection mode will have with other connections in each of the
modes defined in this specification. If such a description is not
provided, the extension connection mode MUST NOT have any
interaction with other connections on the endpoint.
Extension connection modes SHALL NOT be included in the list of modes
in a response to an AuditEndpoint for Capabilities, since the package
will be reported in the list of packages.
6.4 ConnectionParameters
Extension Connection Parameters SHALL include:
* The name and encoding of the connection parameter extension.
* The possible values and encoding of those values that can be
assigned to the connection parameter extension.
* A description of how those values are derived.
Note that the extension connection parameter MUST be included in the
result for an AuditConnection command auditing the connection
parameters.
6.5 DigitMapLetters
Extension Digit Map Letters SHALL include:
* The name and encoding of the extension digit map letter(s).
* A description of the meaning of the extension digit map letter(s).
Note that extension DigitMapLetters in a digit map do not follow the
normal naming conventions for extensions defined in packages. More
specifically the package name and slash ("/") will not be part of the
extension name, thereby forming a flat and limited name space with
potential name clashing.
Therefore, a package SHALL NOT define a digit map letter extension
whose encoding has already been used in another package. If two
packages have used the same encoding for a digit map letter
extension, and those two packages are supported by the same endpoint,
the result of using that digit map letter extension is undefined.
Note that although an extension DigitMapLetter does not include the
package name prefix and slash ("/") as part of the extension name
within a digit map, the package name prefix and slash are included
when the event code for the event that matched the DigitMapLetter is
reported as an observed event. In other words, the digit map just
define the matching rule(s), but the event is still reported like any
other event.
6.6 Events and Signals
The event/signal definition SHALL include the precise name of the
event/signal (i.e., the code used in MGCP), a plain text definition
of the event/signal, and, when appropriate, the precise definition of
the corresponding events/signals, for example the exact frequencies
of audio signals such as dial tones or DTMF tones.
The package description MUST provide, for each event/signal, the
following information:
* The description of the event/signal and its purpose, which SHOULD
include the actual signal that is generated by the client (e.g., xx
ms FSK tone) as well as the resulting user observed result (e.g.,
Message Waiting light on/off).
The event code used for the event/signal.
* The detailed characteristics of the event/signal, such as for
example frequencies and amplitude of audio signals, modulations and
repetitions. Such details may be country specific.
* The typical and maximum duration of the event/signal if applicable.
* If the signal or event can be applied to a connection (across a
media stream), it MUST be indicated explicitly. If no such
indication is provided, it is assumed that the signal or event
cannot be applied to a connection.
For events, the following MUST be provided as well:
* An indication if the event is persistent. By default, events are
not persistent - defining events as being persistent is discouraged
(see Appendix B for a preferred alternative). Note that persistent
events will automatically trigger a Notify when they occur, unless
the Call Agent explicitly instructed the endpoint otherwise. This
not only violates the normal MGCP model, but also assumes the Call
Agent supports the package in question. Such an assumption is
unlikely to hold in general.
* An indication if there is an auditable event-state associated with
the event. By default, events do not have auditable event-states.
* If event parameters are supported, it MUST be stated explicitly.
The precise syntax and semantics of these MUST then be provided
(subject to the grammar provided in Appendix A). It SHOULD also be
specified whether these parameters apply to RequestedEvents,
ObservedEvents, DetectEvents and EventStates. If not specified
otherwise, it is assumed that:
* they do not apply to RequestedEvents,
* they do apply to ObservedEvents,
* they apply in the same way to DetectEvents as they do to
RequestedEvents for a given event parameter,
* they apply in the same way to EventStates as they do to
ObservedEvents for a given event parameter.
* If the event is expected to be used in digit map matching, it
SHOULD explicitly state so. Note that only events with single
letter or digit parameter codes can do this. See Section 2.1.5 for
further details.
For signals, the following MUST be provided as well:
* The type of signal (OO, TO, BR).
* Time-Out signals SHOULD have an indication of the default time-out
value. In some cases, time-out values may be variable (if
dependent on some action to complete such as out-pulsing digits).
* If signal parameters are supported, it MUST be stated explicitly.
The precise syntax and semantics of these MUST then be provided
(subject to the grammar provided in Appendix A).
* Time-Out signals may also indicate whether the "to" parameter is
supported or not as well as what the rounding rules associated with
them are. If omitted from the signal definition, the package-wide
definition is assumed (see Section 6). If the package definition
did not specify this, rounding rules default to the nearest non-
zero second, whereas support for the "to" parameter defaults to
"no" for package version zero, and "yes" for package versions one
and higher.
The following format is RECOMMENDED for defining events and signals
in conformance with the above:
------------------------------------------------------------------
Symbol Definition R S Duration
---------------------------------------------------------------
------------------------------------------------------------------
where:
* Symbol indicates the event code used for the event/signal, e.g.,
"hd".
* Definition gives a brief definition of the event/signal
* R contains an "x" if the event can be detected or one or more of
the following symbols:
- "P" if the event is persistent.
- "S" if the events is an event-state that may be audited.
- "C" if the event can be detected on a connection.
* S contains one of the following if it is a signal:
- "OO" if the signal is On/Off signal.
- "TO" if the signal is a Time-Out signal.
- "BR" if the signal is a Brief signal.
* S also contains:
- "C" if the signal can be applied on a connection.
The table SHOULD then be followed by a more comprehensive description
of each event/signal defined.
6.6.1 Default and Reserved Events
All packages that contain Time-Out type signals contain the operation
failure ("of") and operation complete ("oc") events, irrespective of
whether they are provided as part of the package description or not.
These events are needed to support Time-Out signals and cannot be
overridden in packages with Time-Out signals. They MAY be extended
if necessary, however such practice is discouraged.
If a package without Time-Out signals does contain definitions for
the "oc" and "of" events, the event definitions provided in the
package MAY over-ride those indicated here. Such practice is however
discouraged and is purely allowed to avoid potential backwards
compatibility problems.
It is considered good practice to explicitly mention that the two
events are supported in accordance with their default definitions,
which are as follows:
------------------------------------------------------------------
Symbol Definition R S Duration
---------------------------------------------------------------
oc Operation Complete x
of Operation Failure x
------------------------------------------------------------------
Operation complete (oc): The operation complete event is generated
when the gateway was asked to apply one or several signals of type TO
on the endpoint or connection, and one or more of those signals
completed without being stopped by the detection of a requested event
such as off-hook transition or dialed digit. The completion report
should carry as a parameter the name of the signal that came to the
end of its live time, as in:
O: G/oc(G/rt)
In this case, the observed event occurred because the "rt" signal in
the "G" package timed out.
If the reported signal was applied on a connection, the parameter
supplied will include the name of the connection as well, as in:
O: G/oc(G/rt@0A3F58)
When the operation complete event is requested, it cannot be
parameterized with any event parameters. When the package name is
omitted (which is discouraged) as part of the signal name, the
default package is assumed.
Operation failure (of): The operation failure event is generated
when the endpoint was asked to apply one or several signals of type
TO on the endpoint or connection, and one or more of those signals
failed prior to timing out. The completion report should carry as a
parameter the name of the signal that failed, as in:
O: G/of(G/rt)
In this case a failure occurred in producing the "rt" signal in the
"G" package.
When the reported signal was applied on a connection, the parameter
supplied will include the name of the connection as well, as in:
O: G/of(G/rt@0A3F58)
When the operation failure event is requested, event parameters can
not be specified. When the package name is omitted (which is
discouraged), the default package name is assumed.
6.7 ExtensionParameters
Extension parameter extensions SHALL include:
* The name and encoding of the extension parameter.
* The possible values and encoding of those values that can be
assigned to the extension parameter.
* For each of the commands defined in this specification, whether the
extension parameter is Mandatory, Optional, or Forbidden in
requests as well as responses. Note that extension parameters
SHOULD NOT normally be mandatory.
* A description of the operation of the extension parameter. As part
of this description the default value (if any) if the extension is
omitted in a command MUST be defined. It may be necessary to make
a distinction between the default value before and after the
initial application of the parameter, for example if the parameter
retains its previous value once specified, until explicitly
altered. If default values are not described, then the extension
parameter simply defaults to empty in all commands.
* Whether the extension can be audited in AuditEndpoint and/or
AuditConnection as well as the values returned. If nothing is
specified, then auditing of the extension parameter can only be
done for AuditEndpoint, and the value returned SHALL be the current
value for the extension. Note that this may be empty.
6.8 LocalConnectionOptions
LocalConnectionOptions extensions SHALL include:
* The name and encoding of the LocalConnectionOptions extension.
* The possible values and encoding of those values that can be
assigned to the LocalConnectionOptions extension.
* A description of the operation of the LocalConnectionOptions
extension. As part of this description the following MUST be
specified:
- The default value (if any) if the extension is omitted in a
CreateConnection command.
- The default value if omitted in a ModifyConnection command. This
may be to simply retain the previous value (if any) or to apply
the default value. If nothing is specified, the current value is
retained if possible.
- If Auditing of capabilities will result in the extension being
returned, then a description to that effect as well as with what
possible values and their encoding (note that the package itself
will always be returned). If nothing is specified, the extension
SHALL NOT be returned when auditing capabilities.
Also note, that the extension MUST be included in the result for an
AuditConnection command auditing the LocalConnectionOptions.
6.9 Reason Codes
Extension reason codes SHALL include:
* The number for the reason code. The number MUST be in the range
800 to 899.
* A description of the extension reason code including the
circumstances that leads to the generation of the reason code.
Those circumstances SHOULD be limited to events caused by another
extension defined in the package to ensure the recipient will be
able to interpret the extension reason code correctly.
Note that the extension reason code may have to be provided in the
result for an AuditEndpoint command auditing the reason code.
6.10 RestartMethods
Extension Restart Methods SHALL include:
* The name and encoding for the restart method.
* A description of the restart method including the circumstances
that leads to the generation of the restart method. Those
circumstances SHOULD be limited to events caused by another
extension defined in the package to ensure the recipient will be
able to interpret the extension restart method correctly.
* An indication of whether the RestartDelay parameter is to be used
with the extension. If nothing is specified, it is assumed that it
is not to be used. In that case, RestartDelay MUST be ignored if
present.
* If the restart method defines a service state, the description MUST
explicitly state and describe this. In that case, the extension
restart method can then be provided in the result for an
AuditEndpoint command auditing the restart method.
6.11 Return Codes
Extension Return Codes SHALL include:
* The number for the extension return code. The number MUST be in
the range 800 to 899.
* A description of the extension return code including the
circumstances that leads to the generation of the extension return
code. Those circumstances SHOULD be limited to events caused by
another extension defined in the package to ensure the recipient
will be able to interpret the extension return code correctly.
7. Versions and Compatibility
7.1 Changes from RFC2705
RFC2705 was issued in October 1999, as the last update of draft
version 0.5. This updated document benefits from further
implementation experience. The main changes from RFC2705 are:
* Contains several clarifications, editorial changes and resolution
of known inconsistencies.
* Firmed up specification language in accordance with RFC2119 and
added RFC2119 conventions section.
* Clarified behavior of mixed wild-carding in endpoint names.
* Deleted naming requirement about having first term identify the
physical gateway when the gateway consists of multiple physical
gateways. Also added recommendations on wild-carding naming usage
from the right only, as well as mixed wildcard usage.
* Clarified that synonymous forms and values for endpoint names are
not freely interchangeable.
* Allowed IPv6 addresses in endpoint names.
* Clarified Digit Map matching rules.
* Added missing semantics for symbols used in digit maps.
* Added Timer T description in Digit Maps.
* Added recommendation to support digit map sizes of at least 2048
bytes per endpoint.
* Clarified use of wildcards in several commands.
* Event and Signal Parameters formally defined for events and
signals.
* Persistent events now allowed in base MGCP protocol.
* Added additional detail on connection wildcards.
* Clarified behavior of loopback, and continuity test connection
modes for mixing and multiple connections in those modes.
* Modified BearerInformation to be conditional optional in the
EndpointConfiguration command.
* Clarified "swap audio" action operation for one specific scenario
and noted that operation for other scenarios is undefined.
* Added recommendation that all implementations support PCMU encoding
for interoperability.
* Changed Bandwidth LocalConnectionOptions value from excluding to
including overhead from the IP layer and up for consistency with
SDP.
* Clarified that mode of second connection in a CreateConnection
command will be set to "send/receive".
* Type of service default changed to zero.
* Additional detail on echo cancellation, silence suppression, and
gain control. Also added recommendation for Call Agents not to
specify handling of echo cancellation and gain control.
* Added requirement for a connection to have a
RemoteConnectionDescriptor in order to use the "network loopback"
and "network continuity test" modes.
* Removed procedures and specification for NAS's (will be provided as
package instead).
* Removed procedures and specification for ATM (will be provided as
package instead).
* Added missing optional NotifiedEntity parameter to the
DeleteConnection (from the Call Agent) MGCI command.
* Added optional new MaxMGCPDatagram RequestedInfo code for
AuditEndpoint to enable auditing of maximum size of MGCP datagrams
supported.
* Added optional new PackageList RequestedInfo code for AuditEndpoint
to enable auditing of packages with a package version number.
PackageList parameter also allowed with return code 518
(unsupported package).
* Added missing attributes in Capabilities.
* Clarified that at the expiration of a non-zero restart delay, an
updated RestartInProgress should be sent. Also clarified that a
new NotifiedEntity can only be returned in response to a
RestartInProgress command.
* Added Response Acknowledgement response (return code 000) and
included in three-way handshake.
* ResponseAck parameter changed to be allowed in all commands.
* Added return codes 101, 405, 406, 407, 409, 410, 503, 504, 505,
506, 507, 508, 509, 533, 534, 535, 536, 537, 538, 539, 540, 541,
and defined return codes in range 800-899 to be package specific
return codes. Additional text provided for some return codes and
additional detail on how to handle unknown return codes added.
* Added reason code 903, 904, 905 and defined reason codes 800-899 to
be package specific reason codes.
* Added section clarifying codec negotiation procedure.
* Clarified that resource reservation parameters in a
ModifyConnection command defaults to the current value used.
* Clarified that connection mode is optional in ModifyConnection
commands.
* Corrected LocalConnectionDescriptor to be optional in response to
CreateConnection commands (in case of failure).
* Clarified that quoted-strings are UTF-8 encoded and
interchangeability of quoted strings and unquoted strings.
* Clarified that Transaction Identifiers are compared as numerical
values.
* Clarified bit-ordering for Type Of Service LocalConnectionOptions.
* Clarified the use of RequestIdentifier zero.
* Added example sections for commands, responses, and some call
flows.
* Corrected usage of and requirements for SDP to be strictly RFC2327
compliant.
* Added requirement that all MGCP implementations must support MGCP
datagrams up to at least 4000 bytes. Also added new section on
Maximum Datagram Size, Fragmentation and reassembly.
* Generalized piggybacking retransmission scheme to only state
underlying requirements to be satisfied.
* Clarified the section on computing retransmission timers.
* Clarified operation of long-running transactions, including
provisional responses, retransmissions and failures.
* Enhanced description of provisional responses and interaction with
three-way handshake.
* Enhanced description of fail-over and the role of "notified
entity". An empty "notified entity" has been allowed, although
strongly discouraged.
* Clarified retransmission procedure and removed "wrong key"
considerations from it. Also fixed inconsistencies between Max1
and Max2 retransmission boundaries and the associated flow diagram.
* Updated domain name resolution for retransmission procedure to
incur less overhead when multiple endpoints are retransmitting.
* Removed requirement for in-order delivery of NotificationRequests
response and Notify commands. Notify commands are still delivered
in-order.
* Clarified that activating an embedded Notification Request does not
clear the list of ObservedEvents.
* Defined interactions between disconnected state and notification
state.
* Added section on transactional semantics.
* Defined gateway behavior when multiple interacting transactions are
received.
* Additional details provided on service states. Clarified
relationship between endpoint service states, restart methods, and
associated processing of commands.
* Clarified operation for transitioning from "restart procedure" to
"disconnected state".
* Allowed auditing commands and responses to bypass the "restart" and
"disconnected" procedures.
* Clarified operation of "disconnected procedure" and in particular
the operation of piggy-backed "disconnected" RestartInProgress
messages.
* Added option to aggregate "disconnected" RestartInProgress messages
under certain conditions to reduce message volume.
* Defined additional behavior for endpoints wishing to send commands
while in the "disconnected" state.
* Added new section on Load Control in General which includes two new
error codes (101 and 409) to handle overload.
* Deleted the "Proposed MoveConnection command".
* Removed packages from protocol specification (will be provided in
separate documents instead).
* Package concept formally extended to be primary extension mechanism
now allowing extensions for the following to be defined in packages
as well:
- BearerInformation
- LocalConnectionOptions
- ExtensionParameters
- Connection Modes
- Actions
- Digit Map Letters
- Connection Parameters
- Restart Methods
- Reason Codes
- Return Codes
* Requirements and suggested format for package definitions added.
* Defined "operation complete" and "operation failure" events to be
automatically present in packages with Time-Out signals.
* Deleted list of differences that were prior to RFC2705.
* Added Base Package to deal with quarantine buffer overflow,
ObservedEvents overflow, embedded NotificationRequest failure, and
to enable events to be requested persistently. A new "Message"
command is included as well.
* IANA registration procedures for packages and other extensions
added.
* Updated grammar to fix known errors and support new extensions in a
backwards compatible manner. Added new (optional) PackageList and
MaxMGCPDatagram for auditing. Changed explicit white space rules
in some productions to make grammar more consistent.
* Connection Mode interaction table added.
* Added additional detail on virtual endpoint naming conventions.
Also added suggested gateway endpoint convention and a "Range
Wildcard" option to the Endpoint Naming Conventions.
8. Security Considerations
Security issues are discussed in section 5.
9. Acknowledgements
Special thanks are due to the authors of the original MGCP 1.0
specification: Mauricio Arango, Andrew Dugan, Isaac Elliott,
Christian Huitema, and Scott Picket.
We also want to thank the many reviewers who provided advice on the
design of SGCP and then MGCP, notably Sankar Ardhanari, Francois
Berard, David Auerbach, Bob Biskner, David Bukovinsky, Charles Eckel,
Mario Edini, Ed Guy, Barry Hoffner, Jerry Kamitses, Oren Kudevitzki,
Rajesh Kumar, Troy Morley, Dave Oran, Jeff Orwick, John Pickens, Lou
Rubin, Chip Sharp, Paul Sijben, Kurt Steinbrenner, Joe Stone, and
Stuart Wray.
The version 0.1 of MGCP was heavily inspired by the "Internet
Protocol Device Control" (IPDC) designed by the Technical Advisory
Committee set up by Level 3 Communications. Whole sets of text were
retrieved from the IP Connection Control protocol, IP Media Control
protocol, and IP Device Management. The authors wish to acknowledge
the contribution to these protocols made by Ilya Akramovich, Bob
Bell, Dan Brendes, Peter Chung, John Clark, Russ Dehlinger, Andrew
Dugan, Isaac Elliott, Cary FitzGerald, Jan Gronski, Tom Hess, Geoff
Jordan, Tony Lam, Shawn Lewis, Dave Mazik, Alan Mikhak, Pete
O'Connell, Scott Pickett, Shyamal Prasad, Eric Presworsky, Paul
Richards, Dale Skran, Louise Spergel, David Sprague, Raj Srinivasan,
Tom Taylor and Michael Thomas.
10. References
[1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC2026, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, March 1997.
[3] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", RFC
1889, January 1996.
[4] Schulzrinne, H., "RTP Profile for Audio and Video Conferences
with Minimal Control", RFC1890, January 1996.
[5] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC2327, April 1998.
[6] Handley, M., Perkins, C. and E. Whelan, "Session Announcement
Protocol", RFC2974, October 2000.
[7] Rosenberg, J., Camarillo, G., Johnston, A., Peterson, J.,
Sparks, R., Handley, M., Schulzrinne, H. and E. Schooler,
"Session Initiation Protocol (SIP)", RFC3261, June 2002.
[8] Schulzrinne, H., Rao, A. and R. Lanphier, "Real Time Streaming
Protocol (RTSP)", RFC2326, April 1998.
[9] ITU-T, Recommendation Q.761, "FUNCTIONAL DESCRIPTION OF THE ISDN
USER PART OF SIGNALING SYSTEM No. 7", (Malaga-Torremolinos,
1984; modified at Helsinki, 1993).
[10] ITU-T, Recommendation Q.762, "GENERAL FUNCTION OF MESSAGES AND
SIGNALS OF THE ISDN USER PART OF SIGNALING SYSTEM No. 7",
(MalagaTorremolinos, 1984; modified at Helsinki, 1993).
[11] ITU-T, Recommendation H.323 (02/98), "PACKET-BASED MULTIMEDIA
COMMUNICATIONS SYSTEMS".
[12] ITU-T, Recommendation H.225, "Call Signaling Protocols and Media
Stream Packetization for Packet Based Multimedia Communications
Systems".
[13] ITU-T, Recommendation H.245 (02/98), "CONTROL PROTOCOL FOR
MULTIMEDIA COMMUNICATION".
[14] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC2401, November 1998.
[15] Kent, S. and R. Atkinson, "IP Authentication Header", RFC2402,
November 1998.
[16] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
(ESP)", RFC2406, November 1998.
[17] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC2234, November 1997.
[18] Stevens, W. Richard, "TCP/IP Illustrated, Volume 1, The
Protocols", Addison-Wesley, 1994.
[19] Allman, M., Paxson, V. "On Estimating End-to-End Network Path
Properties", Proc. SIGCOMM'99, 1999.
[20] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
2279, January 1998.
[21] Braden, R., "Requirements for Internet Hosts -- Communication
Layers", STD 3, RFC1122, October 1989.
[22] Bellcore, "LSSGR: Switching System Generic Requirements for Call
Control Using the Integrated Services Digital Network User Part
(ISDNUP)", GR-317-CORE, Issue 2, December 1997.
[23] Narten, T., and Alvestrand H., "Guidelines for Writing an IANA
Considerations Section in RFCs", RFC2434, October 1998.
Appendix A: Formal Syntax Description of the Protocol
In this section, we provide a formal description of the protocol
syntax, following the "Augmented BNF for Syntax Specifications"
defined in RFC2234. The syntax makes use of the core rules defined
in RFC2234, Section 6.1, which are not included here. Furthermore,
the syntax follows the case-sensitivity rules of RFC2234, i.e., MGCP
is case-insensitive (but SDP is not). It should be noted, that ABNF
does not provide for implicit specification of linear white space and
MGCP messages MUST thus follow the explicit linear white space rules
provided in the grammar below. However, in line with general
robustness principles, implementers are strongly encouraged to
tolerate additional linear white space in messages received.
MGCPMessage = MGCPCommand / MGCPResponse
MGCPCommand = MGCPCommandLine 0*(MGCPParameter) [EOL *SDPinformation]
MGCPCommandLine = MGCPVerb 1*(WSP) transaction-id 1*(WSP)
endpointName 1*(WSP) MGCPversion EOL
MGCPVerb = "EPCF" / "CRCX" / "MDCX" / "DLCX" / "RQNT"
/ "NTFY" / "AUEP" / "AUCX" / "RSIP" / extensionVerb
extensionVerb = ALPHA 3(ALPHA / DIGIT) ; experimental starts with X
transaction-id = 1*9(DIGIT)
endpointName = LocalEndpointName "@" DomainName
LocalEndpointName = LocalNamePart 0*("/" LocalNamePart)
LocalNamePart = AnyName / AllName / NameString
AnyName = "$"
AllName = "*"
NameString = 1*(range-of-allowed-characters)
VCHAR except "$", "*", "/", "@"
range-of-allowed-characters = %x21-23 / %x25-29 / %x2B-2E
/ %x30-3F / %x41-7E
DomainName = 1*255(ALPHA / DIGIT / "." / "-") ; as defined
/ "#" number ; in RFC821
/ "[" IPv4address / IPv6address "]" ; see RFC2373
Rewritten to ABNF from RFC821
number = 1*DIGIT
From RFC2373
IPv6address = hexpart [ ":" IPv4address ]
IPv4address = 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT "." 1*3DIGIT
this production, while occurring in RFC2373, is not referenced
IPv6prefix = hexpart "/" 1*2DIGIT
hexpart = hexseq / hexseq "::" [ hexseq ] / "::" [ hexseq ]
hexseq = hex4 *( ":" hex4)
hex4 = 1*4HEXDIG
MGCPversion = "MGCP" 1*(WSP) 1*(DIGIT) "." 1*(DIGIT)
[1*(WSP) ProfileName]
ProfileName = VCHAR *( WSP / VCHAR)
MGCPParameter = ParameterValue EOL
Check infoCode if more parameter values defined
Most optional values can only be omitted when auditing
ParameterValue = ("K" ":" 0*(WSP) [ResponseAck])
/ ("B" ":" 0*(WSP) [BearerInformation])
/ ("C" ":" 0*(WSP) CallId)
/ ("I" ":" 0*(WSP) [ConnectionId])
/ ("N" ":" 0*(WSP) [NotifiedEntity])
/ ("X" ":" 0*(WSP) [RequestIdentifier])
/ ("L" ":" 0*(WSP) [LocalConnectionOptions])
/ ("M" ":" 0*(WSP) ConnectionMode)
/ ("R" ":" 0*(WSP) [RequestedEvents])
/ ("S" ":" 0*(WSP) [SignalRequests])
/ ("D" ":" 0*(WSP) [DigitMap])
/ ("O" ":" 0*(WSP) [ObservedEvents])
/ ("P" ":" 0*(WSP) [ConnectionParameters])
/ ("E" ":" 0*(WSP) ReasonCode)
/ ("Z" ":" 0*(WSP) [SpecificEndpointID])
/ ("Z2" ":" 0*(WSP) SecondEndpointID)
/ ("I2" ":" 0*(WSP) SecondConnectionID)
/ ("F" ":" 0*(WSP) [RequestedInfo])
/ ("Q" ":" 0*(WSP) QuarantineHandling)
/ ("T" ":" 0*(WSP) [DetectEvents])
/ ("RM" ":" 0*(WSP) RestartMethod)
/ ("RD" ":" 0*(WSP) RestartDelay)
/ ("A" ":" 0*(WSP) [Capabilities])
/ ("ES" ":" 0*(WSP) [EventStates])
/ ("PL" ":" 0*(WSP) [PackageList]) ; Auditing only
/ ("MD" ":" 0*(WSP) MaxMGCPDatagram) ; Auditing only
/ (extensionParameter ":" 0*(WSP) [parameterString])
A final response may include an empty ResponseAck
ResponseAck = confirmedTransactionIdRange
*( "," 0*(WSP) confirmedTransactionIdRange )
confirmedTransactionIdRange = transaction-id ["-" transaction-id]
BearerInformation = BearerAttribute 0*("," 0*(WSP) BearerAttribute)
BearerAttribute = ("e" ":" BearerEncoding)
/ (BearerExtensionName [":" BearerExtensionValue])
BearerExtensionName = PackageLCOExtensionName
BearerExtensionValue = LocalOptionExtensionValue
BearerEncoding = "A" / "mu"
CallId = 1*32(HEXDIG)
The audit request response may include a list of identifiers
ConnectionId = 1*32(HEXDIG) 0*("," 0*(WSP) 1*32(HEXDIG))
SecondConnectionID = ConnectionId
NotifiedEntity = [LocalName "@"] DomainName [":" portNumber]
LocalName = LocalEndpointName ; No internal structure
portNumber = 1*5(DIGIT)
RequestIdentifier = 1*32(HEXDIG)
LocalConnectionOptions = LocalOptionValue 0*(WSP)
0*("," 0*(WSP) LocalOptionValue 0*(WSP))
LocalOptionValue = ("p" ":" packetizationPeriod)
/ ("a" ":" compressionAlgorithm)
/ ("b" ":" bandwidth)
/ ("e" ":" echoCancellation)
/ ("gc" ":" gainControl)
/ ("s" ":" silenceSuppression)
/ ("t" ":" typeOfService)
/ ("r" ":" resourceReservation)
/ ("k" ":" encryptiondata)
/ ("nt" ":" ( typeOfNetwork /
supportedTypeOfNetwork))
/ (LocalOptionExtensionName
[":" LocalOptionExtensionValue])
Capabilities = CapabilityValue 0*(WSP)
0*("," 0*(WSP) CapabilityValue 0*(WSP))
CapabilityValue = LocalOptionValue
/ ("v" ":" supportedPackages)
/ ("m" ":" supportedModes)
PackageList = pkgNameAndVers 0*("," pkgNameAndVers)
pkgNameAndVers = packageName ":" packageVersion
packageVersion = 1*(DIGIT)
packetizationPeriod = 1*4(DIGIT) ["-" 1*4(DIGIT)]
compressionAlgorithm = algorithmName 0*(";" algorithmName)
algorithmName = 1*(SuitableLCOCharacter)
bandwidth = 1*4(DIGIT) ["-" 1*4(DIGIT)]
echoCancellation = "on" / "off"
gainControl = "auto" / ["-"] 1*4(DIGIT)
silenceSuppression = "on" / "off"
typeOfService = 1*2(HEXDIG) ; 1 hex only for capabilities
resourceReservation = "g" / "cl" / "be"
encryption parameters are coded as in SDP (RFC2327)
NOTE: encryption key may contain an algorithm as specified in RFC1890
encryptiondata = ( "clear" ":" encryptionKey )
/ ( "base64" ":" encodedEncryptionKey )
/ ( "uri" ":" URItoObtainKey )
/ ( "prompt" ) ; defined in SDP, not usable in MGCP!
encryptionKey = 1*(SuitableLCOCharacter) / quotedString
See RFC2045
encodedEncryptionKey = 1*(ALPHA / DIGIT / "+" / "/" / "=")
URItoObtainKey = 1*(SuitableLCOCharacter) / quotedString
typeOfNetwork = "IN" / "ATM" / "LOCAL" / OtherTypeOfNetwork
Registered with IANA - see RFC2327
OtherTypeOfNetwork = 1*(SuitableLCOCharacter)
supportedTypeOfNetwork = typeOfNetwork *(";" typeOfNetwork)
supportedModes = ConnectionMode 0*(";" ConnectionMode)
supportedPackages = packageName 0*(";" packageName)
packageName = 1*(ALPHA / DIGIT / HYPHEN) ; Hyphen neither first or last
LocalOptionExtensionName = VendorLCOExtensionName
/ PackageLCOExtensionName
/ OtherLCOExtensionName
VendorLCOExtensionName = "x" ("+"/"-") 1*32(SuitableExtLCOCharacter)
PackageLCOExtensionName = packageName "/"
1*32(SuitablePkgExtLCOCharacter)
must not start with "x-" or "x+"
OtherLCOExtensionName = 1*32(SuitableExtLCOCharacter)
LocalOptionExtensionValue = (1*(SuitableExtLCOValChar)
/ quotedString)
*(";" (1*(SuitableExtLCOValChar)
/ quotedString))
Note: No "data" mode.
ConnectionMode = "sendonly" / "recvonly" / "sendrecv"
/ "confrnce" / "inactive" / "loopback"
/ "conttest" / "netwloop" / "netwtest"
/ ExtensionConnectionMode
ExtensionConnectionMode = PkgExtConnectionMode
PkgExtConnectionMode = packageName "/" 1*(ALPHA / DIGIT)
RequestedEvents = requestedEvent 0*("," 0*(WSP) requestedEvent)
requestedEvent = (eventName ["(" requestedActions ")"])
/ (eventName "(" requestedActions ")"
"(" eventParameters ")" )
eventName = [(packageName / "*") "/"]
(eventId / "all" / eventRange
/ "*" / "#") ; for DTMF
["@" (ConnectionId / "$" / "*")]
eventId = 1*(ALPHA / DIGIT / HYPHEN) ; Hyphen neither first nor last
eventRange = "[" 1*(DigitMapLetter / (DIGIT "-" DIGIT) /
(DTMFLetter "-" DTMFLetter)) "]"
DTMFLetter = "A" / "B" / "C" / "D"
requestedActions = requestedAction 0*("," 0*(WSP) requestedAction)
requestedAction = "N" / "A" / "D" / "S" / "I" / "K"
/ "E" "(" EmbeddedRequest ")"
/ ExtensionAction
ExtensionAction = PackageExtAction
PackageExtAction = packageName "/" Action ["(" ActionParameters ")"]
Action = 1*ALPHA
ActionParameters = eventParameters ; May contain actions
NOTE: Should tolerate different order when receiving, e.g., for NCS.
EmbeddedRequest = ( "R" "(" EmbeddedRequestList ")"
["," 0*(WSP) "S" "(" EmbeddedSignalRequest ")"]
["," 0*(WSP) "D" "(" EmbeddedDigitMap ")"] )
/ ( "S" "(" EmbeddedSignalRequest ")"
["," 0*(WSP) "D" "(" EmbeddedDigitMap ")"] )
/ ( "D" "(" EmbeddedDigitMap ")" )
EmbeddedRequestList = RequestedEvents
EmbeddedSignalRequest = SignalRequests
EmbeddedDigitMap = DigitMap
SignalRequests = SignalRequest 0*("," 0*(WSP) SignalRequest )
SignalRequest = eventName [ "(" eventParameters ")" ]
eventParameters = eventParameter 0*("," 0*(WSP) eventParameter)
eventParameter = eventParameterValue
/ eventParameterName "=" eventParameter
/ eventParameterName "(" eventParameters ")"
eventParameterString = 1*(SuitableEventParamCharacter)
eventParameterName = eventParameterString
eventParameterValue = eventParameterString / quotedString
DigitMap = DigitString / "(" DigitStringList ")"
DigitStringList = DigitString 0*( "" DigitString )
DigitString = 1*(DigitStringElement)
DigitStringElement = DigitPosition ["."]
DigitPosition = DigitMapLetter / DigitMapRange
NOTE "X" is now included
DigitMapLetter = DIGIT / "#" / "*" / "A" / "B" / "C" / "D" / "T"
/ "X" / ExtensionDigitMapLetter
ExtensionDigitMapLetter = "E" / "F" / "G" / "H" / "I" / "J" / "K"
/ "L" / "M" / "N" / "O" / "P" / "Q" / "R"
/ "S" / "U" / "V" / "W" / "Y" / "Z"
NOTE "[x]" is now allowed
DigitMapRange = "[" 1*DigitLetter "]"
DigitLetter = *((DIGIT "-" DIGIT) / DigitMapLetter)
ObservedEvents = SignalRequests
EventStates = SignalRequests
ConnectionParameters = ConnectionParameter
0*( "," 0*(WSP) ConnectionParameter )
ConnectionParameter = ( "PS" "=" packetsSent )
/ ( "OS" "=" octetsSent )
/ ( "PR" "=" packetsReceived )
/ ( "OR" "=" octetsReceived )
/ ( "PL" "=" packetsLost )
/ ( "JI" "=" jitter )
/ ( "LA" "=" averageLatency )
/ ( ConnectionParameterExtensionName
"=" ConnectionParameterExtensionValue )
packetsSent = 1*9(DIGIT)
octetsSent = 1*9(DIGIT)
packetsReceived = 1*9(DIGIT)
octetsReceived = 1*9(DIGIT)
packetsLost = 1*9(DIGIT)
jitter = 1*9(DIGIT)
averageLatency = 1*9(DIGIT)
ConnectionParameterExtensionName = VendorCPExtensionName
/ PackageCPExtensionName
VendorCPExtensionName = "X" "-" 2*ALPHA
PackageCPExtensionName = packageName "/" CPName
CPName = 1*(ALPHA / DIGIT / HYPHEN)
ConnectionParameterExtensionValue = 1*9(DIGIT)
MaxMGCPDatagram = 1*9(DIGIT)
ReasonCode = 3DIGIT
[1*(WSP) "/" packageName] ; Only for 8xx
[WSP 1*(%x20-7E)]
SpecificEndpointID = endpointName
SecondEndpointID = endpointName
RequestedInfo = infoCode 0*("," 0*(WSP) infoCode)
infoCode = "B" / "C" / "I" / "N" / "X" / "L" / "M" / "R" / "S"
/ "D" / "O" / "P" / "E" / "Z" / "Q" / "T" / "RC" / "LC"
/ "A" / "ES" / "RM" / "RD" / "PL" / "MD" / extensionParameter
QuarantineHandling = loopControl / processControl
/ (loopControl "," 0*(WSP) processControl )
loopControl = "step" / "loop"
processControl = "process" / "discard"
DetectEvents = SignalRequests
RestartMethod = "graceful" / "forced" / "restart" / "disconnected"
/ "cancel-graceful" / extensionRestartMethod
extensionRestartMethod = PackageExtensionRM
PackageExtensionRM = packageName "/" 1*32(ALPHA / DIGIT / HYPHEN)
RestartDelay = 1*6(DIGIT)
extensionParameter = VendorExtensionParameter
/ PackageExtensionParameter
/ OtherExtensionParameter
VendorExtensionParameter = "X" ("-"/"+") 1*6(ALPHA / DIGIT)
PackageExtensionParameter = packageName "/"
1*32(ALPHA / DIGIT / HYPHEN)
must not start with "x-" or x+"
OtherExtensionParameter = 1*32(ALPHA / DIGIT / HYPHEN)
If first character is a double-quote, then it is a quoted-string
parameterString = (%x21 / %x23-7F) *(%x20-7F) ; first and last must not
; be white space
/ quotedString
MGCPResponse = MGCPResponseLine 0*(MGCPParameter)
*2(EOL *SDPinformation)
MGCPResponseLine = responseCode 1*(WSP) transaction-id
[1*(WSP) "/" packageName] ; Only for 8xx
[WSP responseString] EOL
responseCode = 3DIGIT
responseString = *(%x20-7E)
SuitablePkgExtLCOCharacter = SuitableLCOCharacter
SuitableExtLCOCharacter = DIGIT / ALPHA / "+" / "-" / "_" / "&"
/ "!" / "'" / "" / "=" / "#" / "?"
/ "." / "$" / "*" / "@" / "[" / "]"
/ "^" / "`" / "{" / "}" / "~"
SuitableLCOCharacter = SuitableExtLCOCharacter / "/"
SuitableExtLCOValChar = SuitableLCOCharacter / ":"
VCHAR except """, "(", ")", ",", and "="
SuitableEventParamCharacter = %x21 / %x23-27 / %x2A-2B
/ %x2D-3C / %x3E-7E
NOTE: UTF8 encoded
quotedString = DQUOTE 0*(quoteEscape / quoteChar) DQUOTE
quoteEscape = DQUOTE DQUOTE
quoteChar = (%x00-21 / %x23-FF)
EOL = CRLF / LF
HYPHEN = "-"
See RFC2327 for proper SDP grammar instead.
SDPinformation = SDPLine CRLF *(SDPLine CRLF) ; see RFC2327
SDPLine = 1*(%x01-09 / %x0B / %x0C / %x0E-FF) ; for proper def.
Appendix B: Base Package
Package name: B
Version: 0
The MGCP specification defines a base package which contains a set of
events and extension parameters that are of general use to the
protocol. Although not required, it is highly RECOMMENDED to support
this package as it provides important functionality for the base
protocol.
B.1 Events
The table below lists the events:
------------------------------------------------------------------
Symbol Definition R S Duration
---------------------------------------------------------------
enf(##) embedded RQNT failure x
oef observed events full x
qbo quarantine buffer overflow x
------------------------------------------------------------------
The events are defined as follows:
Embedded NotificationRequest failure (enf):
The Embedded NotificationRequest Failure (enf) event is generated
when an embedded Notification Request failure occurs. When the
event is requested, it should be as part of the Embedded
NotificationRequest itself. When the event is reported, it may be
parameterized with an error code (see Section 2.4) detailing the
error that occurred. When requested, it cannot be parameterized.
Observed events full (oef):
The event is generated when the endpoint is unable to accumulate
any more events in the list of ObservedEvents. If this event
occurs, and it is not used to trigger a Notify, subsequent events
that should have been added to the list will be lost.
Quarantine buffer overflow (qbo):
The event is generated when the quarantine buffer overflows and one
or more events have been lost.
B.2 Extension Parameters
B.2.1 PersistentEvents
PersistentEvents: A list of events that the gateway is requested to
detect and report persistently. The parameter is optional but can be
provided in any command where the DetectEvents parameter can be
provided. The initial default value of the parameter is empty. When
the parameter is omitted from a command, it retains its current
value. When the parameter is provided, it completely replaces the
current value. Providing an event in this list, is similar (but
preferable) to defining that particular event as being persistent.
The current list of PersistentEvents will implicitly apply to the
current as well as subsequent NotificationRequests, however no glare
detection etc. will be performed (similarly to DetectEvents). If an
event provided in this list is included in a RequestedEvents list,
the action and event parameters used in the RequestedEvents will
replace the action and event parameters associated with the event in
the PersistentEvents list for the life of the RequestedEvents list,
after which the PersistentEvents action and event parameters are
restored. Events with event states requested through this parameter
will be included in the list of EventStates if audited.
PersistentEvents can also be used to detect events on connections.
Use of the "all connections" wildcard is straightforward, whereas
using PersistentEvents with one or more specific connections must be
considered carefully. Once the connection in question is deleted, a
subsequent NotificationRequest without a new PersistentEvents value
will fail (error code 515 - incorrect connection-id, is RECOMMENDED),
as it implicitly refers to the deleted connection.
The parameter generates the relevant error codes from the base
protocol, e.g., error code 512 if an unknown event is specified.
The PersistentEvents parameter can be audited, in which case it will
return its current value. Auditing of RequestedEvents is not
affected by this extension, i.e., events specified in this list are
not automatically reported when auditing RequestedEvents.
The parameter name for PersistentEvents is "PR" and it is defined by
the production:
PersistentEvents = "PR" ":" 0*WSP [RequestedEvents]
The following example illustrates the use of the parameter:
B/PR: L/hd(N), L/hf(N), L/hu(N), B/enf, B/oef, B/qbo
which instructs the endpoint to persistently detect and report off-
hook, hook-flash, and on-hook. It also instructs the endpoint to
persistently detect and report Embedded Notification Request failure,
Observed events full, and Quarantine buffer overflow.
B.2.2 NotificationState
NotificationState is a RequestedInfo parameter that can be audited
with the AuditEndpoint command. It can be used to determine if the
endpoint is in the notification state or not.
The parameter is forbidden in any command. In responses, it is a
valid response parameter for AuditEndpoint only.
It is defined by the following grammar:
NotificationState = "NS" ":" 0*WSP NotificationStateValue
NotificationStateValue = "ns" / "ls" / "o"
It is requested as part of auditing by including the parameter code
in RequestedInfo, as in:
F: B/NS
The response parameter will contain the value "ns" if the endpoint is
in the "notification state", the value "ls" if the endpoint is in the
"lockstep state" (i.e., waiting for an RQNT after a response to a
NTFY has been received when operating in "step" mode), or the value
"o" otherwise, as for example:
B/NS: ns
B.3 Verbs
MGCP packages are not intended to define new commands, however an
exception is made in this case in order to add an important general
capability currently missing, namely the ability for the gateway to
send a generic message to the Call Agent.
The definition of the new command is:
ReturnCode
<-- Message(EndpointId
[, ...])
EndpointId is the name for the endpoint(s) in the gateway which is
issuing the Message command. The identifier MUST be a fully
qualified endpoint identifier, including the domain name of the
gateway. The local part of the endpoint name MUST NOT use the "any
of" wildcard.
The only parameter specified in the definition of the Message command
is the EndpointId, however, it is envisioned that extensions will
define additional parameters to be used with the Message command.
Such extensions MUST NOT alter or otherwise interfere with the normal
operation of the basic MGCP protocol. They may however define
additional capabilities above and beyond that provided by the basic
MGCP protocol. For example, an extension to enable the gateway to
audit the packages supported by the Call Agent could be defined,
whereas using the Message command as an alternative way of reporting
observed events would be illegal, as that would alter the normal MGCP
protocol behavior.
In order to not interfere with normal MGCP operation, lack of a
response to the Message command MUST NOT lead the endpoint to become
disconnected. The endpoint(s) MUST be prepared to handle this
transparently and continue normal processing unaffected.
If the endpoint(s) receive a response indicating that the Call Agent
does not support the Message command, the endpoint(s) MUST NOT send a
Message command again until the current "notified entity" has
changed. Similarly, if the endpoint(s) receive a response indicating
that the Call Agent does not support one or more parameters in the
Message command, the endpoint(s) MUST NOT send a Message command with
those parameters again until the current "notified entity" has
changed.
The Message command is encoded as MESG, as shown in the following
example:
MESG 1200 aaln/1@rgw.whatever.net MGCP 1.0
Appendix C: IANA Considerations
C.1 New MGCP Package Sub-Registry
The IANA has established a new sub-registry for MGCP packages under
http://www.iana.org/assignments/mgcp-packages.
Packages can be registered with the IANA according to the following
procedure:
The package MUST have a unique string name which MUST NOT start with
the two characters "x-" or "x+".
The package title, name, and version (zero assumed by default) MUST
be registered with IANA as well as a reference to the document that
describes the package. The document MUST have a stable URL and MUST
be contained on a public web server.
Packages may define one or more Extension Digit Map Letters, however
these are taken from a limited and flat name space. To prevent name
clashing, IANA SHALL NOT register a package that defines an Extension
Digit Map Letter already defined in another package registered by
IANA. To ease this task, such packages SHALL contain the line
"Extension Digit Map Letters: " followed by a list of the Extension
Digit Map Letters defined in the package at the beginning of the
package definition.
A contact name, e-mail and postal address for the package MUST be
provided. The contact information SHALL be updated by the defining
organization as necessary.
Finally, prior to registering a package, the IANA MUST have a
designated expert [23] review the package. The expert reviewer will
send e-mail to the IANA on the overall review determination.
C.2 New MGCP Package
This document defines a new MGCP Base Package in Appendix B, which
has been registered by IANA.
C.3 New MGCP LocalConnectionOptions Sub-Registry
The IANA has established a new sub-registry for MGCP
LocalConnectionOptions under http://www.iana.org/assignments/mgcp-
localconnectionoptions.
Packages are the preferred extension mechanism, however for backwards
compatibility, local connection options beyond those provided in this
specification can be registered with IANA. Each such local
connection option MUST have a unique string name which MUST NOT start
with "x-" or "x+". The local connection option field name and
encoding name MUST be registered with IANA as well as a reference to
the document that describes the local connection option. The
document MUST have a stable URL and MUST be contained on a public web
server.
A contact name, e-mail and postal address for the local connection
option MUST be provided. The contact information SHALL be updated by
the defining organization as necessary.
Finally, prior to registering a LocalConnectionOption, the IANA MUST
have a designated expert [23] review the LocalConnectionOption. The
expert reviewer will send e-mail to the IANA on the overall review
determination.
Appendix D: Mode Interactions
An MGCP endpoint can establish one or more media streams. These
streams are either incoming (from a remote endpoint) or outgoing
(generated at the handset microphone). The "connection mode"
parameter establishes the direction and generation of these streams.
When there is only one connection to an endpoint, the mapping of
these streams is straightforward; the handset plays the incoming
stream over the handset speaker and generates the outgoing stream
from the handset microphone signal, depending on the mode parameter.
However, when several connections are established to an endpoint,
there can be many incoming and outgoing streams. Depending on the
connection mode used, these streams may interact differently with
each other and the streams going to/from the handset.
The table below describes how different connections SHALL be mixed
when one or more connections are concurrently "active". An active
connection is here defined as a connection that is in one of the
following modes:
* "send/receive"
* "send only"
* "receive only"
* "conference"
Connections in "network loopback", "network continuity test", or
"inactive" modes are not affected by connections in the "active"
modes. The Table uses the following conventions:
* Ai is the incoming media stream from Connection A
* Bi is the incoming media stream from Connection B
* Hi is the incoming media stream from the Handset Microphone
* Ao is the outgoing media stream to Connection A
* Bo is the outgoing media stream to Connection B
* Ho is the outgoing media stream to the Handset earpiece
* NA indicates no stream whatsoever (assuming there are no signals
applied on the connection)
"netw" in the following table indicates either "netwloop" or
"netwtest" mode.
-------------------------------------------------------------
Connection A Mode
-----------------------------------------------------
sendonlyrecvonlysendrecvconfrnceinactive netw
------------------------------------------------------------
Send Ao=Hi Ao=NA Ao=Hi Ao=Hi Ao=NA Ao=Ai
Conly Bo=Hi Bo=Hi Bo=Hi Bo=Hi Bo=Hi Bo=Hi
o Ho=NA Ho=Ai Ho=Ai Ho=Ai Ho=NA Ho=NA
n-----------------------------------------------------------
nrecv Ao=NA Ao=Hi Ao=Hi Ao=NA Ao=Ai
eonly Bo=NA Bo=NA Bo=NA Bo=NA Bo=NA
c Ho=Ai+BiHo=Ai+BiHo=Ai+Bi Ho=Bi Ho=Bi
t-----------------------------------------------------------
isend Ao=Hi Ao=Hi Ao=NA Ao=Ai
orecv Bo=Hi Bo=Hi Bo=Hi Bo=Hi
n Ho=Ai+BiHo=Ai+Bi Ho=Bi Ho=Bi
-----------------------------------------------------------
Bconf Ao=Hi+Bi Ao=NA Ao=Ai
rnce Bo=Hi+Ai Bo=Hi Bo=Hi
M Ho=Ai+Bi Ho=Bi Ho=Bi
o-----------------------------------------------------------
dInac Ao=NA Ao=Ai
etive Bo=NA Bo=NA
Ho=NA Ho=NA
-----------------------------------------------------------
netw Ao=Ai
Bo=Bi
Ho=NA
-------------------------------------------------------------
If there are three or more "active" connections they will still
interact as defined in the table above with the outgoing media
streams mixed for each interaction (union of all streams). If
internal resources are used up and the streams cannot be mixed, the
gateway MUST return an error (error code 403 or 502, not enough
resources, are RECOMMENDED).
Appendix E: Endpoint Naming Conventions
The following sections provide some RECOMMENDED endpoint naming
conventions.
E.1 Analog Access Line Endpoints
The string "aaln", should be used as the first term in a local
endpoint name for analog access line endpoints. Terms following
"aaln" should follow the physical hierarchy of the gateway so that if
the gateway has a number of RJ11 ports, the local endpoint name could
look like the following:
aaln/#
where "#" is the number of the analog line (RJ11 port) on the
gateway.
On the other hand, the gateway may have a number of physical plug-in
units, each of which contain some number of RJ11 ports, in which
case, the local endpoint name might look like the following:
aaln/<unit #>/#
where <unit #> is the number of the plug in unit in the gateway and
"#" is the number of the analog line (RJ11 port) on that unit.
Leading zeroes MUST NOT be used in any of the numbers ("#") above.
E.2 Digital Trunks
The string "ds" should be used for the first term of digital
endpoints with a naming convention that follows the physical and
digital hierarchy such as:
ds/<unit-type1>-<unit #>/<unit-type2>-<unit #>/.../<channel #>
where: <unit-type> identifies the particular hierarchy level. Some
example values of <unit-type> are: "s", "su", "oc3", "ds3", "e3",
"ds2", "e2", "ds1", "e1" where "s" indicates a slot number and "su"
indicates a sub-unit within a slot. Leading zeroes MUST NOT be used
in any of the numbers ("#") above.
The <unit #> is a decimal number which is used to reference a
particular instance of a <unit-type> at that level of the hierarchy.
The number of levels and naming of those levels is based on the
physical hierarchy within the media gateway.
E.3 Virtual Endpoints
Another type of endpoint is one that is not associated with a
physical interface (such as an analog or digital endpoint). This
type of endpoint is called a virtual endpoint and is often used to
represent some DSP resources that gives the endpoint some capability.
Examples are announcement, IVR or conference bridge devices. These
devices may have multiple instances of DSP functions so that a
possible naming convention is:
<virtual-endpoint-type>/<endpoint-#>
where <virtual-endpoint-type> may be some string representing the
type of endpoint (such as "ann" for announcement server or "cnf" for
conference server) and <endpoint-#> would identify a particular
virtual endpoint within the device. Leading zeroes MUST NOT be used
in the number ("#") above. If the physical hierarchy of the server
includes plug-in DSP cards, another level of hierarchy in the local
endpoint name may be used to describe the plug in unit.
A virtual endpoint may be created as the result of using the "any of"
wildcard. Similarly, a virtual endpoint may cease to exist once the
last connection on the virtual endpoint is deleted. The definition
of the virtual endpoint MUST detail both of these ASPects.
When a <virtual-endpoint-type> creates and deletes virtual endpoints
automatically, there will be cases where no virtual endpoints exist
at the time a RestartInProgress command is to be issued. In such
cases, the gateway SHOULD simply use the "all of" wildcard in lieu of
any specific <endpoint-#> as in, e.g.:
ann/*@mygateway.whatever.net
If the RestartInProgress command refers to all endpoints in the
gateway (virtual or not), the <virtual-endpoint-id> can be omitted as
in, e.g.:
*@mygateway.whatever.net
Commands received by the gateway will still have to refer to an
actual endpoint (possibly created by that command by use of the "any
of" wildcard) in order for the command to be processed though.
E.4 Media Gateway
MGCP only defines operation on endpoints in a media gateway. It may
be beneficial to define an endpoint that represents the gateway
itself as opposed to the endpoints managed by the gateway.
Implementations that wish to do so should use the local endpoint name
"mg" (for media gateway) as in:
mg@mygateway.whatever.net
Note that defining such an endpoint does not change any of the
protocol semantics, i.e., the "mg" endpoint and other endpoints
(e.g., digital trunks) in the gateway are still independent endpoints
and MUST be treated as such. For example, RestartInProgress commands
MUST still be issued for all endpoints in the gateway as usual.
E.5 Range Wildcards
As described in Section 2.1.2, the MGCP endpoint naming scheme
defines the "all of" and "any of" wildcards for the individual terms
in a local endpoint name. While the "all of" wildcard is very useful
for reducing the number of messages, it can by definition only be
used when we wish to refer to all instances of a given term in the
local endpoint name. Furthermore, in the case where a command is to
be sent by the gateway to the Call Agent, the "all of" wildcard can
only be used if all of the endpoints named by it have the same
"notified entity". Implementations that prefer a finer-grained
wildcarding scheme can use the range wildcarding scheme described
here.
A range wildcard is defined as follows:
RangeWildcard = "[" NumericalRange *( "," NumericalRange ) "]"
NumericalRange = 1*(DIGIT) [ "-" 1*(DIGIT) ]
Note that white space is not permitted. Also, since range wildcards
use the character "[" to indicate the start of a range, the "["
character MUST NOT be used in endpoint names that use range
wildcards. The length of a range wildcard SHOULD be bounded to a
reasonably small value, e.g., 128 characters.
Range wildcards can be used anywhere an "all of" wildcard can be
used. The semantics are identical for the endpoints named. However,
it MUST be noted, that use of the range wildcarding scheme requires
support on both the gateway and the Call Agent. Therefore, a gateway
MUST NOT assume that it's Call Agent supports range wildcarding and
vice versa. In practice, this typically means that both the gateway
and Call Agent will need to be provisioned consistently in order to
use range wildcards. Also, if a gateway or Call Agent using range
wildcards receives an error response that could indicate a possible
endpoint naming problem, they MUST be able to automatically revert to
not using range wildcards.
The following examples illustrates the use of range wildcards:
ds/ds1-1/[1-12]
ds/ds1-1/[1,3,20-24]
ds/ds1-[1-2]/*
ds/ds3-1/[1-96]
The following example illustrates how to use it in a command:
RSIP 1204 ds/ds3-1/[1-96]@tgw-18.whatever.net MGCP 1.0
RM: restart
RD: 0
Appendix F: Example Command Encodings
This appendix provides examples of commands and responses shown with
the actual encoding used. Examples are provided for each command.
All commentary shown in the commands and responses is optional.
F.1 NotificationRequest
The first example illustrates a NotificationRequest that will ring a
phone and look for an off-hook event:
RQNT 1201 aaln/1@rgw-2567.whatever.net MGCP 1.0
N: ca@ca1.whatever.net:5678
X: 0123456789AC
R: l/hd(N)
S: l/rg
The response indicates that the transaction was successful:
200 1201 OK
The second example illustrates a NotificationRequest that will look
for and accumulate an off-hook event, and then provide dial-tone and
accumulate digits according to the digit map provided. The "notified
entity" is set to "ca@ca1.whatever.net:5678", and since the
SignalRequests parameter is empty (it could have been omitted as
well), all currently active TO signals will be stopped. All events
in the quarantine buffer will be processed, and the list of events to
detect in the "notification" state will include fax tones in addition
to the "requested events" and persistent events:
RQNT 1202 aaln/1@rgw-2567.whatever.net MGCP 1.0
N: ca@ca1.whatever.net:5678
X: 0123456789AC
R: L/hd(A, E(S(L/dl),R(L/oc, L/hu, D/[0-9#*T](D))))
D: (0T00T#xxxxxxx*xx91xxxxxxxxxx9011x.T)
S:
Q: process
T: G/ft
The response indicates that the transaction was successful:
200 1202 OK
F.2 Notify
The example below illustrates a Notify message that notifies an off-
hook event followed by a 12-digit number beginning with "91". A
transaction identifier correlating the Notify with the
NotificationRequest it results from is included. The command is sent
to the current "notified entity", which typically will be the actual
value supplied in the NotifiedEntity parameter, i.e.,
"ca@ca1.whatever.net:5678" - a failover situation could have changed
this:
NTFY 2002 aaln/1@rgw-2567.whatever.net MGCP 1.0
N: ca@ca1.whatever.net:5678
X: 0123456789AC
O: L/hd,D/9,D/1,D/2,D/0,D/1,D/8,D/2,D/9,D/4,D/2,D/6,D/6
The Notify response indicates that the transaction was successful:
200 2002 OK
F.3 CreateConnection
The first example illustrates a CreateConnection command to create a
connection on the endpoint specified. The connection will be part of
the specified CallId. The LocalConnectionOptions specify that G.711
mu-law will be the codec used and the packetization period will be 10
ms. The connection mode will be "receive only":
CRCX 1204 aaln/1@rgw-2567.whatever.net MGCP 1.0
C: A3C47F21456789F0
L: p:10, a:PCMU
M: recvonly
The response indicates that the transaction was successful, and a
connection identifier for the newly created connection is therefore
included. A session description for the new connection is included
as well - note that it is preceded by an empty line.
200 1204 OK
I: FDE234C8
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=-
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 0
The second example illustrates a CreateConnection command containing
a notification request and a RemoteConnectionDescriptor:
CRCX 1205 aaln/1@rgw-2569.whatever.net MGCP 1.0
C: A3C47F21456789F0
L: p:10, a:PCMU
M: sendrecv
X: 0123456789AD
R: L/hd
S: L/rg
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=-
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 0
The response indicates that the transaction failed, because the phone
was already off-hook. Consequently, neither a connection-id nor a
session description is returned:
401 1205 Phone off-hook
Our third example illustrates the use of the provisional response and
the three-way handshake. We create another connection and
acknowledge the previous response received by using the response
acknowledgement parameter:
CRCX 1206 aaln/1@rgw-2569.whatever.net MGCP 1.0
K: 1205
C: A3C47F21456789F0
L: p:10, a:PCMU
M: inactive
v=0
o=- 25678 753849 IN IP4 128.96.41.1
s=-
c=IN IP4 128.96.41.1
t=0 0
m=audio 3456 RTP/AVP 0
A provisional response is returned initially:
100 1206 Pending
I: DFE233D1
v=0
o=- 4723891 7428910 IN IP4 128.96.63.25
s=-
c=IN IP4 128.96.63.25
t=0 0
m=audio 3456 RTP/AVP 0
A little later, the final response is received:
200 1206 OK
K:
I: DFE233D1
v=0
o=- 4723891 7428910 IN IP4 128.96.63.25
s=-
c=IN IP4 128.96.63.25
t=0 0
m=audio 3456 RTP/AVP 0
The Call Agent acknowledges the final response as requested:
000 1206
and the transaction is complete.
F.4 ModifyConnection
The first example shows a ModifyConnection command that simply sets
the connection mode of a connection to "send/receive" - the "notified
entity" is set as well:
MDCX 1209 aaln/1@rgw-2567.whatever.net MGCP 1.0
C: A3C47F21456789F0
I: FDE234C8
N: ca@ca1.whatever.net
M: sendrecv
The response indicates that the transaction was successful:
200 1209 OK
In the second example, we pass a session description and include a
notification request with the ModifyConnection command. The endpoint
will start playing ring-back tones to the user:
MDCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0
C: A3C47F21456789F0
I: FDE234C8
M: recvonly
X: 0123456789AE
R: L/hu
S: G/rt
v=0
o=- 4723891 7428910 IN IP4 128.96.63.25
s=-
c=IN IP4 128.96.63.25
t=0 0
m=audio 3456 RTP/AVP 0
The response indicates that the transaction was successful:
200 1206 OK
F.5 DeleteConnection (from the Call Agent)
In this example, the Call Agent simply instructs the gateway to
delete the connection "FDE234C8" on the endpoint specified:
DLCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0
C: A3C47F21456789F0
I: FDE234C8
The response indicates success, and that the connection was deleted.
Connection parameters for the connection are therefore included as
well:
250 1210 OK
P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27, LA=48
F.6 DeleteConnection (from the gateway)
In this example, the gateway sends a DeleteConnection command to the
Call Agent to instruct it that a connection on the specified endpoint
has been deleted. The ReasonCode specifies the reason for the
deletion, and Connection Parameters for the connection are provided
as well:
DLCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0
C: A3C47F21456789F0
I: FDE234C8
E: 900 - Hardware error
P: PS=1245, OS=62345, PR=780, OR=45123, PL=10, JI=27, LA=48
The Call Agent sends a success response to the gateway:
200 1210 OK
F.7 DeleteConnection (multiple connections from the Call Agent)
In the first example, the Call Agent instructs the gateway to delete
all connections related to call "A3C47F21456789F0" on the specified
endpoint:
DLCX 1210 aaln/1@rgw-2567.whatever.net MGCP 1.0
C: A3C47F21456789F0
The response indicates success and that the connection(s) were
deleted:
250 1210 OK
In the second example, the Call Agent instructs the gateway to delete
all connections related to all of the endpoints specified:
DLCX 1210 aaln/*@rgw-2567.whatever.net MGCP 1.0
The response indicates success:
250 1210 OK
F.8 AuditEndpoint
In the first example, the Call Agent wants to learn what endpoints
are present on the gateway specified, hence the use of the "all of"
wild-card for the local portion of the endpoint-name:
AUEP 1200 *@rgw-2567.whatever.net MGCP 1.0
The gateway indicates success and includes a list of endpoint names:
200 1200 OK
Z: aaln/1@rgw-2567.whatever.net
Z: aaln/2@rgw-2567.whatever.net
In the second example, the capabilities of one of the endpoints is
requested:
AUEP 1201 aaln/1@rgw-2567.whatever.net MGCP 1.0
F: A
The response indicates success and the capabilities as well. Two
codecs are supported, however with different capabilities.
Consequently two separate capability sets are returned:
200 1201 OK
A: a:PCMU, p:10-100, e:on, s:off, v:L;S, m:sendonly;
recvonly;sendrecv;inactive;netwloop;netwtest
A: a:G729, p:30-90, e:on, s:on, v:L;S, m:sendonly;
recvonly;sendrecv;inactive;confrnce;netwloop
Note that the carriage return in the Capabilities lines are shown for
formatting reasons only - they are not permissible in a real
implementation.
In the third example, the Call Agent audits several types of
information for the endpoint:
AUEP 2002 aaln/1@rgw-2567.whatever.net MGCP 1.0
F: R,D,S,X,N,I,T,O,ES
The response indicates success:
200 2002 OK
R: L/hu,L/oc(N),D/[0-9](N)
D:
S: L/vmwi(+)
X: 0123456789B1
N: [128.96.41.12]
I: 32F345E2
T: G/ft
O: L/hd,D/9,D/1,D/2
ES: L/hd
The list of requested events contains three events. Where no package
name is specified, the default package is assumed. The same goes for
actions, so the default action - Notify - must therefore be assumed
for the "L/hu" event. The omission of a value for the "digit map"
means the endpoint currently does not have a digit map. There are
currently no active time-out signals, however the OO signal "vmwi" is
currently on and is consequently included - in this case it was
parameterized, however the parameter could have been excluded. The
current "notified entity" refers to an IP-address and only a single
connection exists for the endpoint. The current value of
DetectEvents is "G/ft", and the list of ObservedEvents contains the
four events specified. Finally, the event-states audited reveals
that the phone was off-hook at the time the transaction was
processed.
F.9 AuditConnection
The first example shows an AuditConnection command where we audit the
CallId, NotifiedEntity, LocalConnectionOptions, Connection Mode,
LocalConnectionDescriptor, and the Connection Parameters:
AUCX 2003 aaln/1@rgw-2567.whatever.net MGCP 1.0
I: 32F345E2
F: C,N,L,M,LC,P
The response indicates success and includes information for the
RequestedInfo:
200 2003 OK
C: A3C47F21456789F0
N: ca@ca1.whatever.net
L: p:10, a:PCMU
M: sendrecv
P: PS=395, OS=22850, PR=615, OR=30937, PL=7, JI=26, LA=47
v=0
o=- 4723891 7428910 IN IP4 128.96.63.25
s=-
c=IN IP4 128.96.63.25
t=0 0
m=audio 1296 RTP/AVP 0
In the second example, we request to audit RemoteConnectionDescriptor
and LocalConnectionDescriptor:
AUCX 1203 aaln/2@rgw-2567.whatever.net MGCP 1.0
I: FDE234C8
F: RC,LC
The response indicates success, and includes information for the
RequestedInfo. In this case, no RemoteConnectionDescriptor exists,
hence only the protocol version field is included for the
RemoteConnectionDescriptor:
200 1203 OK
v=0
o=- 4723891 7428910 IN IP4 128.96.63.25
s=-
c=IN IP4 128.96.63.25
t=0 0
m=audio 1296 RTP/AVP 0
v=0
F.10 RestartInProgress
The first example illustrates a RestartInProgress message sent by an
gateway to inform the Call Agent that the specified endpoint will be
taken out-of-service in 300 seconds:
RSIP 1200 aaln/1@rgw-2567.whatever.net MGCP 1.0
RM: graceful
RD: 300
The Call Agent's response indicates that the transaction was
successful:
200 1200 OK
In the second example, the RestartInProgress message sent by the
gateway informs the Call Agent, that all of the gateway's endpoints
are being placed in-service in 0 seconds, i.e., they are currently in
service. The restart delay could have been omitted as well:
RSIP 1204 *@rgw-2567.whatever.net MGCP 1.0
RM: restart
RD: 0
The Call Agent's response indicates success, and furthermore provides
the endpoints in question with a new "notified entity":
200 1204 OK
N: CA-1@whatever.net
Alternatively, the command could have failed with a new "notified
entity" as in:
521 1204 OK
N: CA-1@whatever.net
In that case, the command would then have to be retried in order to
satisfy the "restart procedure", this time going to Call Agent "CA-
1@whatever.net".
Appendix G: Example Call Flows
The message flow tables in this section use the following
abbreviations:
* rgw = Residential Gateway
* ca = Call Agent
* n+ = step 'n' is repeated one or more times
Note that any use of upper and lower case within the text of the
messages is to aid readability and is not in any way a requirement.
The only requirement involving case is to be case insensitive at all
times.
G.1 Restart
G.1.1 Residential Gateway Restart
The following table shows a message sequence that might occur when a
call agent (ca) is contacted by two independent residential gateways
(rgw1 and rgw2) which have restarted.
Table F.1: Residential Gateway Restart
---------------------------------------------------------------------
step# usr1 rgw1 ca rgw2 usr2
================================================================
1 rsip ->
<- ack
----------------------------------------------------------------
2 <- auep
ack ->
----------------------------------------------------------------
3+ <- rqnt
ack ->
----------------------------------------------------------------
4 <- rsip
ack ->
----------------------------------------------------------------
5 auep ->
<- ack
----------------------------------------------------------------
6+ rqnt ->
<- ack
---------------------------------------------------------------------
Step 1 - RestartInProgress (rsip) from rgw1 to ca
rgw1 uses DNS to determine the domain name of ca and send to the
default port of 2727. The command consists of the following:
rsip 1 *@rgw1.whatever.net mgcp 1.0
rm: restart
The "*" is used to inform ca that all endpoints of rgw1 are being
restarted, and "restart" is specified as the restart method. The
Call Agent "ca" acknowledges the command with an acknowledgement
message containing the transaction-id (in this case 1) for the
command. It sends the acknowledgement to rgw1 using the same port
specified as the source port for the rsip. If none was indicated, it
uses the default port of 2727.
200 1 ok
A response code is mandatory. In this case, "200", indicates "the
requested transaction was executed normally". The response string is
optional. In this case, "ok" is included as an additional
description.
Step 2 - AuditEndpoint (auep) from ca to rgw1
The command consists of the following:
auep 153 *@rgw1.whatever.net mgcp 1.0
The "*" is used to request audit information from rgw1 of all its
endpoints. rgw1 acknowledges the command with an acknowledgement
message containing the transaction-id (in this case 153) of the
command, and it includes a list of its endpoints. In this example,
rgw1 has two endpoints, aaln/1 and aaln/2.
200 153 ok
Z: aaln/1@rgw1.whatever.net
Z: aaln/2@rgw1.whatever.net
Once it has the list of endpoint ids, ca may send individual
AuditEndpoint commands in which the "*" is replaced by the id of the
given endpoint. As its response, rgw1 would replace the endpoint id
list returned in the example with the info requested for the
endpoint. This optional message exchange is not shown in this
example.
Step 3 - NotificationRequest (rqnt) from ca to each endpoint of rgw1
In this case, ca sends two rqnts, one for aaln/1:
rqnt 154 aaln/1@rgw1.whatever.net mgcp 1.0
r: l/hd(n)
x: 3456789a0
and a second for aaln/2:
rqnt 155 aaln/2@rgw1.whatever.net mgcp 1.0
r: l/hd(n)
x: 3456789a1
Note that in the requested events parameter line, the event is fully
specified as "l/hd", i.e., with the package name, in order to avoid
any potential ambiguity. This is the recommended behavior. For the
sake of clarity, the action, which in this case is to Notify, is
explicitly specified by including the "(n)". If no action is
specified, Notify is assumed as the default regardless of the event.
If any other action is desired, it must be stated explicitly.
The expected response from rgw1 to these requests is an
acknowledgement from aaln/1 as follows:
200 154 ok
and from aaln/2:
200 155 ok
Step 4 RestartInProgress (rsip) from rgw2 to ca
rsip 0 *@rgw2.whatever.net mgcp 1.0
rm: restart
followed by the acknowledgement from ca:
200 0 ok
Step 5 - AuditEndpoint (auep) from ca to rgw2
auep 156 *@rgw2.whatever.net mgcp 1.0
followed by an acknowledgement from rgw2:
200 156 ok
z: aaln/1@rgw2.whatever.net
z: aaln/2@rgw2.whatever.net
Step 6 - NotificationRequest (rqnt) from ca to each endpoint of rgw2
rqnt 157 aaln/1@rgw2.whatever.net mgcp 1.0
r: l/hd(n)
x: 3456789a2
followed by:
rqnt 158 aaln/2@rgw2.whatever.net mgcp 1.0
r: l/hd(n)
x: 3456789a3
with rgw2 acknowledging for aaln/1:
200 157 ok
and for aaln/2:
200 158 ok
G.1.2 Call Agent Restart
The following table shows the message sequence which occurs when a
call agent (ca) restarts. How it determines the address information
of the gateways, in this case rgw1 and rgw2, is not covered in this
document. For interoperability, it is RECOMMENDED to provide the
ability to configure the call agent to send AUEP (*) to specific
addresses and ports.
Table F.2: Residential Gateway Restart
---------------------------------------------------------------------
# usr1 rgw1 ca rgw2 usr2
================================================================
1 <- auep
ack ->
----------------------------------------------------------------
2+ <- rqnt
ack ->
----------------------------------------------------------------
3 auep ->
<- ack
----------------------------------------------------------------
4+ rqnt ->
<- ack
---------------------------------------------------------------------
Step 1 - AuditEndpoint (auep) from ca to rgw1
The command consists of the following:
auep 0 *@rgw1.whatever.net mgcp 1.0
The "*" is used to request audit information from rgw1 of all its
endpoints. rgw1 acknowledges the command with an acknowledgement
message containing the transaction id (in this case 0) of the
command, and it includes a list of its endpoints. In this example,
rgw1 has two endpoints, aaln/1 and aaln/2.
200 0 ok
z: aaln/1@rgw1.whatever.net
z: aaln/2@rgw1.whatever.net
Once it has the list of endpoint ids, ca may send individual
AuditEndpoint commands in which the "*" is replaced by the id of the
given endpoint. As its response, rgw1 would replace the endpoint id
list returned in the example with the info requested for the
endpoint. This optional message exchange is not shown in this
example.
Step 2 - NotificationRequest (rqnt) off-hook from ca to rgw1
In this case, ca sends two rqnts, one for aaln/1:
rqnt 1 aaln/1@rgw1.whatever.net mgcp 1.0
r: l/hd(n)
x: 234567890
and a second for aaln/2:
rqnt 2 aaln/2@rgw1.whatever.net mgcp 1.0
r: l/hd(n)
x: 234567891
The expected response from rgw1 to these requests is an
acknowledgement from aaln/1 as follows:
200 1 ok
and from aaln/2:
200 2 ok
Step 3 - AuditEndpoint (auep) from ca to rgw2
auep 3 *@rgw2.whatever.net mgcp 1.0
followed by an acknowledgement from rgw2:
200 3 ok
z: aaln/1@rgw2.whatever.net
z: aaln/2@rgw2.whatever.net
Step 4 - NotificationRequest (rqnt) from ca to each endpoint of rgw2
rqnt 4 aaln/1@rgw2.whatever.net mgcp 1.0
r: l/hd(n)
x: 234567892
followed by:
rqnt 5 aaln/2@rgw2.whatever.net mgcp 1.0
r: l/hd(n)
x: 234567893
with rgw2 acknowledging for aaln/1:
200 4 ok
and for aaln/2:
200 5 ok
G.2 Connection Creation
G.2.1 Residential Gateway to Residential Gateway
The following table shows the message sequence which occurs when a
user (usr1) makes a call through a residential gateway (rgw1) to a
user served by another residential gateway (rgw2). This example
illustrates the communication between the residential gateways and
the call agent (ca) only. The local name of the endpoints in this
example is aaln/1 for both gateways, and references within the
description of the steps to rgw1 and rgw2 can be assumed to refer to
aaln/1 of rgw1 and aaln/1 of rgw2. Note that this is only an example
and is not the only legal call scenario.
Table F.3: Residential Gateway Connection Creation
---------------------------------------------------------------------
# usr1 rgw1 ca rgw2 usr2
================================================================
1 offhook -> ntfy ->
<- ack
----------------------------------------------------------------
2 <- dialtone <- rqnt
ack ->
----------------------------------------------------------------
3 digits -> ntfy ->
<- ack
----------------------------------------------------------------
4 <- rqnt
ack ->
----------------------------------------------------------------
5 <- recvonly <- crcx
ack ->
----------------------------------------------------------------
6 crcx -> sendrcv ->
<- ack
----------------------------------------------------------------
7 <- recvonly <- mdcx
ack ->
----------------------------------------------------------------
8 <- ringback <- rqnt
ack ->
----------------------------------------------------------------
9 rqnt -> ringing ->
<- ack
----------------------------------------------------------------
10 <- ntfy <- offhook
ack ->
----------------------------------------------------------------
11 rqnt ->
<- ack
----------------------------------------------------------------
12 <- rqnt
ack ->
----------------------------------------------------------------
13 <- sendrcv <- mdcx
ack ->
---------------------------------------------------------------------
Step 1 - Notify (ntfy) offhook from rgw1 to ca
This ntfy is the result of usr1 going offhook and assumes ca had
previously sent an rqnt with RequestId "445678944" to rgw1 requesting
notification in the event of an offhook:
ntfy 12 aaln/1@rgw1.whatever.net mgcp 1.0
o: l/hd
x: 445678944
Acknowledgement from ca:
200 12 ok
Step 2 - Request Notification (rqnt) for digits from ca to rgw1
Request rgw1 to notify if on-hook and collect digits according to the
digit map, and to provide dialtone:
rqnt 1057 aaln/1@rgw1.whatever.net mgcp 1.0
r: l/hu(n), d/[0-9#*T](d)
s: l/dl
x: 445678945
d: 5xxx
Acknowledgement from rgw1:
200 1057 ok
Step 3 - Notify (ntfy) digits from rgw1 to ca
ntfy 13 aaln/1@rgw1.whatever.net mgcp 1.0
o: d/5, d/0, d/0, d/1
x: 445678945
Acknowledgement from ca:
200 13 ok
Step 4 - Request Notification (rqnt) from ca to rgw1
Request rgw1 to notify in the event of an on-hook transition:
rqnt 1058 aaln/1@rgw1.whatever.net mgcp 1.0
r: l/hu(n)
x: 445678946
Acknowledgement from rgw1:
200 1058 ok
Step 5 - Create Connection (crcx) from ca to rgw1
Request a new connection on rgw1 with the specified local connection
options, including 20 msec as the packetization period, G.711 mu-law
as the codec, and receive only as the mode:
crcx 1059 aaln/1@rgw1.whatever.net mgcp 1.0
c: 9876543210abcdef
l: p:20, a:PCMU
m: recvonly
Acknowledgement from rgw1 that a new connection, "456789fedcba5", has
been created, followed by a blank line and then the SDP parameters:
200 1059 ok
i: 456789fedcba5
v=0
o=- 23456789 98765432 IN IP4 192.168.5.7
s=-
c=IN IP4 192.168.5.7
t=0 0
m=audio 6058 RTP/AVP 0
Step 6 - Create Connection (crcx) from ca to rgw2
Request a new connection on rgw2. The request includes the session
description returned by rgw1 such that a two way connection can be
initiated:
crcx 2052 aaln/1@rgw2.whatever.net mgcp 1.0
c: 9876543210abcdef
l: p:20, a:PCMU
m: sendrecv
v=0
o=- 23456789 98765432 IN IP4 192.168.5.7
s=-
c=IN IP4 192.168.5.7
t=0 0
m=audio 6058 RTP/AVP 0
Acknowledgement from rgw2 that a new connection, "67890af54c9", has
been created; followed by a blank line and then the SDP parameters:
200 2052 ok
i: 67890af54c9
v=0
o=- 23456889 98865432 IN IP4 192.168.5.8
s=-
c=IN IP4 192.168.5.8
t=0 0
m=audio 6166 RTP/AVP 0
Step 7 - Modify Connection (mdcx) from ca to rgw1
Request rgw1 to modify the existing connection, "456789fedcba5", to
use the session description returned by rgw2 establishing a half
duplex connection which, though not used in this example, could be
used to provide usr1 with in band ringback tone, announcements, etc:
mdcx 1060 aaln/1@rgw1.whatever.net mgcp 1.0
c: 9876543210abcdef
i: 456789fedcba5
l: p:20, a:PCMU
M: recvonly
v=0
o=- 23456889 98865432 IN IP4 192.168.5.8
s=-
c=IN IP4 192.168.5.8
t=0 0
m=audio 6166 RTP/AVP 0
Acknowledgement from rgw1:
200 1060 ok
Step 8 - Request Notification (rqnt) from ca for rgw1 to provide
ringback
Request rgw1 to notify in the event of an on-hook transition, and
also to provide ringback tone:
rqnt 1061 aaln/1@rgw1.whatever.net mgcp 1.0
r: l/hu(n)
s: g/rt
x: 445678947
Acknowledgement from rgw1:
200 1061 ok
Step 9 - Request Notification (rqnt) from ca to rgw2 to provide
ringing
Request rgw2 to continue to look for offhook and provide ringing:
rqnt 2053 aaln/1@rgw2.whatever.net mgcp 1.0
r: l/hd(n)
s: l/rg
x: 445678948
Acknowledgement from rgw2:
200 2053 ok
Step 10 - Notify (ntfy) offhook from rgw2 to ca
ntfy 27 aaln/1@rgw2.whatever.net mgcp 1.0
o: l/hd
x: 445678948
Acknowledgement from ca:
200 27 ok
Step 11 - Request Notification (rqnt) of on-hook from ca to rgw2
rqnt 2054 aaln/1@rgw2.whatever.net mgcp 1.0
r: l/hu(n)
x: 445678949
Acknowledgement from rgw2:
200 2054 ok
Step 12 - Request Notification (rqnt) of on-hook from ca to rgw1
rqnt 1062 aaln/1@rgw1.whatever.net mgcp 1.0
r: l/hu(n)
x: 445678950
Acknowledgement from rgw1:
200 1062 ok
Step 13 - Modify Connection (mdcx) from ca to rgw1
Request rgw1 to modify the existing connection, "456789fedcba5", to
sendrecv such that a full duplex connection is initiated:
mdcx 1063 aaln/1@rgw1.whatever.net mgcp 1.0
c: 9876543210abcdef
i: 456789fedcba5
m: sendrecv
Acknowledgement from rgw1:
200 1063 ok
G.3 Connection Deletion
G.3.1 Residential Gateway to Residential Gateway
The following table shows the message sequence which occurs when a
user (usr2) initiates the deletion of an existing connection on a
residential gateway (rgw2) with a user served by another residential
gateway (rgw1). This example illustrates the communication between
the residential gateways and the call agent (ca) only. The local
name of the endpoints in this example is aaln/1 for both gateways,
and references within the description of the steps to rgw1 and rgw2
can be assumed to refer to aaln/1 of rgw1 and aaln/1 of rgw2.
Table F.4: Residential Gateway Connection Deletion
---------------------------------------------------------------------
# usr1 rgw1 ca rgw2 usr2
================================================================
1 <- ntfy <- on-hook
ack ->
----------------------------------------------------------------
2 dlcx ->
<- ack
----------------------------------------------------------------
3 <- dlcx
ack ->
----------------------------------------------------------------
4 rqnt ->
<- ack
----------------------------------------------------------------
5 on-hook -> ntfy ->
<- ack
----------------------------------------------------------------
6 <- rqnt
ack ->
---------------------------------------------------------------------
Step 1 - Notify (ntfy) offhook from rgw1 to ca
This ntfy is the result of usr2 going on-hook and assumes that ca had
previously sent an rqnt to rgw2 requesting notification in the event
of an on-hook (see end of Connection Creation sequence):
ntfy 28 aaln/1@rgw2.whatever.net mgcp 1.0
o: l/hu
x: 445678949
Acknowledgement from ca:
200 28 ok
Step 2 - Delete Connection (dlcx) from ca to rgw2
Requests rgw2 to delete the connection "67890af54c9":
dlcx 2055 aaln/1@rgw1.whatever.net mgcp 1.0
c: 9876543210abcdef
i: 67890af54c9
Acknowledgement from rgw2. Note the response code of "250" meaning
"the connection was deleted":
250 2055 ok
Step 3 - Delete Connection (dlcx) from ca to rgw1
Requests rgw1 to delete the connection "456789fedcba5":
dlcx 1064 aaln/1@rgw1.whatever.net mgcp 1.0
c: 9876543210abcdef
i: 456789fedcba5
Acknowledgement from rgw1:
250 1064 ok
Step 4 - NotificationRequest (rqnt) from ca to rgw2
Requests rgw2 to notify ca in the event of an offhook transition:
rqnt 2056 aaln/1@rgw2.whatever.net mgcp 1.0
r: l/hd(n)
x: 445678951
Acknowledgement from rgw2:
200 2056 ok
Step 5 - Notify (ntfy) on-hook from rgw1 to ca
Notify ca that usr1 at rgw1 went back on-hook:
ntfy 15 aaln/1@rgw1.whatever.net mgcp 1.0
o: l/hu
x: 445678950
Acknowledgement from ca:
200 15 ok
Step 6 - NotificationRequest (rqnt) offhook from ca to rgw1
Requests rgw1 to notify ca in the event of an offhook transition:
rqnt 1065 aaln/1@rgw1.whatever.net mgcp 1.0
r: l/hd(n)
x: 445678952
Acknowledgement from rgw1:
200 1065 ok
Authors' Addresses
Flemming Andreasen
Cisco Systems
499 Thornall Street, 8th Floor
Edison, NJ 08837
EMail: fandreas@cisco.com
Bill Foster
Cisco Systems
771 Alder Drive
Milpitas, CA 95035
EMail: bfoster@cisco.com
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